Ink composition, inkjet recording method, printed material, process for producing lithographic printing plate, and lithographic printing plate

ABSTRACT

An ink composition is provided that includes (a) an onium salt, a counteranion thereof having a volume of 1,000 Å 3  or greater and (b) a cationically polymerizable compound. There is also provided an inkjet recording method that includes (a′) a step of discharging the ink composition onto a recording medium and (b′) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation. There are also provided a printed material obtained by the inkjet recording method, a process for producing a lithographic printing plate employing the ink composition, and a lithographic printing plate obtained by the production process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ink composition suitably used for inkjet recording, an inkjet recording method, and a printed material employing same; furthermore, it relates to a lithographic printing plate obtained using the ink, and a process for producing a lithographic printing plate. More particularly, it relates to an ink composition suitable for inkjet recording that cures with high sensitivity upon exposure to radiation, can form a high quality image, and has good storage stability, an inkjet recording method, a printed material employing same, a lithographic printing plate obtained using the ink, and a process for producing a lithographic printing plate.

2. Description of the Related Art

With regard to an image recording method for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic system, sublimation type and melt type thermal transfer systems, an inkjet system, etc. In the electrophotographic system, a process of forming an electrostatic latent image on a photosensitive drum by electrically charging and exposing is required, and the system is complicated; as a result, there is the problem that the production cost is high. With regard to the thermal transfer system, although the equipment is inexpensive, due to the use of an ink ribbon there is the problem that the running cost is high and waste material is generated. On the other hand, with regard to the inkjet system, the equipment is inexpensive and, since an image is formed directly on a recording medium by discharging an ink only on a required image area, the ink can be used efficiently and the running cost is low. Furthermore, there is little noise and it is excellent as an image recording system.

An ink composition that can be cured by exposure to radiation such as ultraviolet rays and, in particular, an inkjet recording ink (radiation curing type inkjet recording ink) are required to have sufficiently high sensitivity and provide a high image quality. By achieving higher-sensitivity, a large number of benefits are provided, such as high curability toward radiation, a reduction in power consumption, longer lifetime due to a decrease in the load on a radiation generator, and prevention of formation of low molecular weight material originating from insufficient curing. Furthermore, higher sensitivity particularly improves the cure strength of an image formed using the ink composition and, in particular, the inkjet recording ink, particularly for the formation of a lithographic printing plate, and high plate life can be obtained.

On the other hand, as a cationic polymerization initiator, compounds disclosed by inventions described in, for example, JP-A-6-184170, JP-A-9-202873, and JP-A-9-221652 (JP-A denotes a Japanese unexamined patent application publication) can be cited, and there is a desire for an ink composition in which a cationically polymerizable compound has high polymerization efficiency and the ink composition has excellent sensitivity toward radiation.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention, which has been accomplished under the above-mentioned circumstances, to provide an ink composition that cures with high sensitivity by exposure to actinic radiation, can form a high quality image, has excellent adhesion to a recording medium, has good storage stability, and can prolong the life of an ink head, and an inkjet recording method employing the ink composition.

It is another object of the present invention to provide a printed material and a lithographic printing plate obtained using an ink composition that has excellent storage stability and can be cured with high sensitivity by exposure to actinic radiation, and a process for producing a lithographic printing plate.

The above-mentioned objects can be accomplished by (1), (6), and (8) to (10) below. (2) to (5) and (7), which are preferred embodiments, are also shown below.

-   (1) An ink composition comprising (a) an onium salt, a counteranion     thereof having a volume of 1,000 Å³ or greater, and (b) a     cationically polymerizable compound, -   (2) the ink composition according to (1) above, wherein the ink     composition comprises a colorant, -   (3) the ink composition according to (2) above, wherein the colorant     is a pigment or an oil-soluble dye, -   (4) the ink composition according to (3) above, wherein the     oil-soluble dye has an oxidation potential of 1.0 V (vs SCE) or     greater, -   (5) the ink composition according to any one of (1) to (4) above,     wherein it is for inkjet recording, -   (6) an inkjet recording method comprising (a) a step of discharging     an ink composition onto a recording medium, and (b) a step of curing     the ink composition by irradiating the discharged ink composition     with actinic radiation, wherein the ink composition is the ink     composition according to any one of (1) to (5) above, -   (7) the inkjet recording method according to (6) above, wherein the     actinic radiation is ultraviolet radiation emitted by a light     emitting diode that has a light emission peak wavelength in the     range of 350 to 420 nm and generates ultraviolet radiation whose     maximum illumination intensity on the surface of a recording medium     is 10 to 2,000 mW/cm², -   (8) a printed material recorded by the inkjet recording method     according to either (6) or (7), -   (9) a process for producing a lithographic printing plate, the     process comprising (a) a step of discharging the ink composition     according to any one of (1) to (5) above onto a hydrophilic support,     and (b) a step of curing the ink composition by irradiating the     discharged ink composition with actinic radiation so as to form a     hydrophobic image on the hydrophilic support from the cured ink     composition, and -   (10) a lithographic printing plate produced by the process for     producing a lithographic printing plate according to (9) above.

DETAILED DESCRIPTION OF THE INVENTION

The ink composition of the present invention (hereinafter, also simply called an ‘ink’) comprises (a) an onium salt, a counteranion thereof having a volume of 1,000 Å³ (cubic angstroms) or greater, and (b) a cationically polymerizable compound.

The present invention is explained in detail below.

(1) Components Contained in Ink Composition

The ink composition of the present invention can be cured by radiation, and may comprise (a) an onium salt, a counteranion thereof having a volume of 1,000 Å³ or greater, and (b) a cationically polymerizable compound, and as necessary (c) a colorant, (d) a sensitizing dye, (e) a cosensitizer, (f) another polymerizable compound, (g) another polymerization initiator, and (h) other components.

The ‘radiation’ referred to in the present invention is not particularly limited as long as it is actinic radiation that can provide energy that enables an initiating species to be generated in the ink composition when irradiated, and broadly covers α rays, γ rays, X rays, ultraviolet rays, visible light, and an electron beam; among these ultraviolet rays and an electron beam are preferable from the viewpoint of curing sensitivity and the availability of equipment, and ultraviolet rays are particularly preferable. The ink composition of the present invention is therefore preferably an ink composition that can be cured by exposure to ultraviolet rays as radiation.

(a) Onium Salt, a Counteranion Thereof Having a Volume of 1,000 Å³ or Greater

The ink composition of the present invention comprises (a) an onium salt, a counteranion thereof having a volume of 1,000 Å³ or greater (hereinafter, also called a ‘specific onium salt’).

The specific onium salt (a) of the ink composition of the present invention has the function of generating a cationic polymerization active species upon exposure to radiation, and initiating and making polymerization of (b) a cationically polymerizable compound proceed.

The specific onium salt (a) that can be used in the present invention may be used singly or in a combination of two or more types.

The counteranion of the specific onium salt is not particularly limited as long as it has a volume of 1,000 Å³ or greater, and the counteranion is preferably an organoboron anion, organosulfonate anion, organocarboxylate anion, polymeric sulfonate (poly)anion, or polymeric carboxylate (poly)anion that has a volume of 1,000 Å³ or greater, more preferably an organoboron anion, organosulfonate anion, or organocarboxylate anion that has an volume of 1,000 Å³ or greater, and yet more preferably an organoboron anion having a volume of 1,000 Å³ or greater. In the case of a polyanion, the volume of the counteranion is defined as the quotient obtained by dividing the entire volume of the polyanion by the number of anions.

The counteranion preferably contains in its structure at least one electron-withdrawing group, preferably contains at least one perfluoroalkyl group or perfluoroaryl group, and particularly preferably contains at least one perfluoroalkyl group. With regard to the position at which the electron-withdrawing group is bonded to the counteranion, it may preferably be bonded directly to the anionic atom or bonded to the anionic atom via an unsaturated bond or an aromatic ring.

The volume of the counteranion is 1,000 Å³ (1.00 nm³) or greater, preferably 1,200 Å³ or greater, and more preferably 1,500 Å³ or greater.

The volume of the counteranion is preferably no greater than 5.00×10³ Å³, and more preferably no greater than 3.00×10³ Å³.

The volume of the counteranion referred to in the present invention denotes the volume occupied by a van der Waals sphere based on the van der Waals radius of atoms forming the counteranion.

A method for calculating the volume of a counteranion is not particularly limited; a known method may be used, and examples thereof include a method in which a molecular structure is identified by radiation structural analysis of a crystal using X-ray structural analysis or neutron beam analysis and calculation is carried out, and a method in which an optimized structure of a molecule is determined by a semi-empirical molecular orbital method, a non-empirical molecular orbital method, or a molecular force field method and calculation is carried out. As a method for determining molecular volume from the molecular structure, a known method may be used, and examples thereof include a space lattice division convergence method and an atomic group contribution method.

As a specific method for calculating the volume of a counteranion, calculation is carried out as follows using WinMOPAC calculation software (manufactured by Fujitsu).

The most stable conformation obtained by molecular force field calculation (MM3) is subjected to molecular orbital calculation (AM1) to give a van der Waals surface, and the value of a surface volume (using an accessible surface parameter) is determined.

Preferred specific examples of the counteranion having a volume of 1,000 Å³ or greater include counteranions having the structures shown in An-1 to An-12 below.

The onium ion (countercation) of the specific onium salt that can be used in the present invention may be an onium ion of a known onium salt; examples thereof include onium ions such as iodonium, sulfonium, phosphonium, diazonium, ammonium, pyridinium, quinolinium, acridinium, oxonium, selenonium, and arsonium, and among these, onium ions such as iodonium, sulfonium, phosphonium, diazonium, quinolinium, and acridinium are preferable.

Examples of the countercation of the specific onium salt include cations such as onium ions of onium salts of group 15 to 17 elements described in JP-A-6-184170, etc., diazonium ions of diazonium salts described in S. I. Schlesinger, Photogr. Sci. Eng., 18, 387 (1974), T. S. Bal et al., Polymer, 21, 423 (1980), etc., ammonium ions of ammonium salts described in U.S. Pat. Nos. 4,069,055, 4,069,056, and Re 27,992, JP-A-3-140140, etc., phosphonium ions of phosphonium salts described in D. C. Necker et al., Macromolecules, 17, 2468 (1984), C. S. Wen et al., The Proc. Conf. Rad. Curing ASIA, p. 478 Tokyo, Oct (1988), U.S. Pat. Nos. 4,069,055 and 4,069,056, JP-A-9-202873, etc., iodonium ions of iodonium salts described in J. V. Crivello et al., Macromolecules, 10 (6), 1307 (1977), Chem. & Eng. News, Nov. 28, p. 31 (1988), European Patent Nos. 104,143, 339,049, and 410,201, JP-A-2-150848, JP-A-2-296514, etc., sulfonium ions of sulfonium salts described in J. V. Crivello et al., Polymer J.17,73 (1985), J. V. Crivello et al., J. Org. Chem., 43, 3055 (1978), W. R. Watt et al., J. Polymer Sci., Polymer Chem. Ed., 22, 1789 (1984), J. V. Crivello et al., Polymer Bull.,14, 279 (1985), J. V. Crivello et al., Macromolecules, 14(5), 1141 (1981), J. V. Crivello et al., J. Polymer Sci., Polymer Chem. Ed., 17, 2877 (1979), European Patent Nos. 370,693, 161,811, 410,201, 339,049, 233,567, 297,443, and 297,442, U.S. Pat. Nos. 3,902,114, 4,933,377, 4,760,013, 4,734,444, and 2,833,827, German Patent Nos. 2,904,626, 3,604,580, and 3,604,581, JP-A-7-28237, JP-A-8-27102, etc., quinolinium ions of quinolinium salts described in JP-A-9-221652, selenonium ions of selenonium salts described in J. V. Crivello et al., Macromolecules, 10 (6), 1307 (1977), J. V. Crivello et al., J. Polymer Sci., Polymer Chem. Ed., 17, 1047 (1979), etc., and arsonium ions of arsonium salts described in C. S. Wen et al., The Proc. Conf. Rad. Curing ASIA, p. 478 Tokyo, Oct (1988), etc., but should not be construed as being limited thereto.

Furthermore, preferred examples of the countercation of (a) the specific onium salt include cations having structures represented by Formulae (I) to (VI) below.

In Formulae (I) to (VI) above, R¹ to R³ independently denote an aryl group, R⁴ to R⁶ independently denote an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a cyclic hydrocarbon group, or a heterocyclic group, R⁷ to R¹¹ independently denote an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a cyclic hydrocarbon group, a heterocyclic group, an alkoxy group, or an aryloxy group, and R¹² to R¹⁷ independently denote a hydrogen atom, a halogen atom, or a monovalent organic group.

The alkyl groups denoted by R⁴ to R¹¹ preferably have 1 to 30 carbons, more preferably 1 to 20 carbons, and particularly preferably 1 to 8 carbons, and may be a straight chain and may have a substituent.

The alkenyl groups denoted by R⁴ to R¹¹ preferably have 2 to 30 carbons, more preferably 2 to 20 carbons, and particularly preferably 2 to 8 carbons, and may further have a substituent.

The alkynyl groups denoted by R⁴ to R¹¹ preferably have 2 to 30 carbons, more preferably 2 to 20 carbons, and particularly preferably 2 to 8 carbons, and may further have a substituent.

The aryl groups denoted by R¹ to R¹¹ preferably have 6 to 30 carbons, more preferably 6 to 20 carbons, and particularly preferably 6 to 10 carbons, and may further have a substituent.

The cyclic hydrocarbon groups denoted by R⁴ to R¹¹ preferably have 3 to 30 carbons, more preferably 3 to 20 carbons, and particularly preferably 3 to 10 carbons, and may further have a substituent.

The heterocyclic groups denoted by R⁴ to R¹¹ preferably have 4 to 30 carbons, more preferably 4 to 20 carbons, and particularly preferably 4 to 10 carbons, and may further have a substituent. Furthermore, the hetero atom contained in the heterocyclic group is preferably a nitrogen atom, an oxygen atom, or a sulfur atom.

The alkoxy groups denoted by R⁷ to R¹¹ preferably have 1 to 30 carbons, more preferably 1 to 20 carbons, and particularly preferably 1 to 8 carbons. Furthermore, the alkoxy groups may have a substituent described below, and the alkyl moiety of the alkoxy groups may be an alkenyl group, an alkynyl group, a cyclic hydrocarbon group, or a heterocyclic group other than an aromatic group.

The aryloxy groups denoted by R⁷ to R¹¹ preferably have 6 to 30 carbons, more preferably 6 to 20 carbons, and particularly preferably 6 to 10 carbons. Furthermore, the aryloxy groups may have a substituent described below, and the aryl moiety of the aryloxy groups may be an aromatic heterocyclic group.

In Formula (II), R² and R³ may be bonded to each other to form a ring if this is possible.

In Formula (III), two or more of R⁴ to R⁶ may be bonded to each other to form a ring if this is possible.

In Formula (IV), two or more of R⁷ to R¹⁰ may be bonded to each other to form a ring if this is possible.

In Formula (V), two or more of R¹¹ to R¹⁴ may be bonded to each other to form a ring if this is possible.

In Formula (VI), two or more of R¹⁵ to R¹⁷ may be bonded to each other to form a ring if this is possible.

With regard to substituents that may be possessed by the alkyl group, the alkenyl group, the alkynyl group, the aryl group, the hydrocarbon group, the heterocyclic group, the alkoxy group, or the aryloxy group, a monovalent non-metallic atomic group other than hydrogen is used, and preferred examples thereof include a halogen atom (—F, —Br, —Cl, —I), a hydroxyl group, an alkyl group, an aryl group, an alkenyl group, an alkynyl group, an alkoxy group, an aryloxy group, a mercapto group, an alkylthio group, an arylthio group, an alkyldithio group, an aryidithio group, an amino group, an N-alkylamino group, an N,N-dialkylamino group, an N-arylamino group, an N,N-diarylamino group, an N-alkyl-N-arylamino group, an acyloxy group, a carbamoyloxy group, an N-alkylcarbamoyloxy group, an N-arylcarbamoyloxy group, an N,N-dialkylcarbamoyloxy group, an N,N-diarylcarbamoyloxy group, an N-alkyl-N-arylcarbamoyloxy group, an alkylsulfoxy group, an arylsulfoxy group, an acylthio group,

an acylamino group, an N-alkylacylamino group, an N-arylacylamino group, a ureido group, an N′-alkylureido group, an N,N′-dialkylureido group, an N′-arylureido group, an N′,N′-diarylureido group, an N′-alkyl-N′-arylureido group, an N-alkylureido group, an N-arylureido group, an N′-alkyl-N-alkylureido group, an N′-alkyl-N-arylureido group, an N,N′-dialkyl-N-alkylureido group, an N,N′-dialkyl-N-arylureido group, an N′-aryl-N-alkylureido group, an N′-aryl-N-arylureido group, an N′,N′-diaryl-N-alkylureido group, an N,N′-diaryl-N-arylureido group, an N′-alkyl-N′-aryl-N-alkylureido group, an N′-alkyl-N′-aryl-N-arylureido group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, an N-alkyl-N-alkoxycarbonylamino group, an N-alkyl-N-aryloxycarbonylamino group, an N-aryl-N-alkoxycarbonylamino group, an N-aryl-N-aryloxycarbonylamino group,

a formyl group, an acyl group, a carboxyl group and conjugate base group thereof (hereinafter, called a carboxylate), an alkoxycarbonyl group, an aryloxycarbonyl group, a carbamoyl group, an N-alkylcarbamoyl group, an N,N-dialkylcarbamoyl group, an N-arylcarbamoyl group, an N,N-diarylcarbamoyl group, an N-alkyl-N-arylcarbamoyl group, an alkylsulfinyl group, an arylsulfinyl group, an alkylsulfonyl group, an arylsulfonyl group, a sulfo group (—SO₃H) and conjugate base group thereof (hereinafter, called a sulfonate group), an alkoxysulfonyl group, an aryloxysulfonyl group, a sulfinamoyl group, an N-alkylsulfinamoyl group, an N,N-dialkylsulfinamoyl group, an N-arylsulfinamoyl group, an N,N-diarylsulfinamoyl group, an N-alkyl-N-arylsulfinamoyl group, a sulfamoyl group, an N-alkylsulfamoyl group, an N, N-dialkylsulfamoyi group, an N-arylsulfamoyl group, an N,N-diarylsulfamoyl group, an N-alkyl-N-arylsulfamoyl group, an N-acylsulfamoyl group and conjugate base group thereof, an N-alkylsulfonylsulfamoyl group (—SO₂NHSO₂(alkyl)) and conjugate base group thereof, an N-arylsulfonylsulfamoyl group (—SO₂NHSO₂(aryl)) and conjugate base group thereof, an N-alkylsulfonylcarbamoyl group (—CONHSO₂(alkyl)) and conjugate base group thereof, an N-arylsulfonylcarbamoyl group (—CONHSO₂(aryl)) and conjugate base group thereof,

a silyl group, an alkoxysilyl group (—Si(O-alkyl)₃), an aryloxysilyl group (—Si(O-allyl)₃), a hydroxysilyl group (—Si(OH)₃) and conjugate base group thereof, a phosphono group (—PO₃H₂) and conjugate base group thereof (hereinafter, called a phosphonate group), a dialkylphosphono group (—PO₃(alkyl)₂), a diarylphosphono group (—PO₃(aryl)₂), an alkylarylphosphono group (—PO₃(alkyl)(aryl)), a monoalkylphosphono group (—PO₃H(alkyl)) and conjugate base group thereof (hereinafter, called an alkylphosphonate group), a monoarylphosphono group (—PO₃H(aryl)) and conjugate base group thereof (hereinafter, called an arylphosphonate group), a phosphonooxy group (—OPO₃H₂) and conjugate base group thereof (hereinafter, called a phosphonatooxy group), a dialkylphosphonooxy group (—OPO₃(alkyl)₂), a diarylphosphonooxy group (—OPO₃(aryl)₂), an alkylarylphosphonooxy group (—OPO₃(alkyl)(aryl)), a monoalkylphosphonooxy group (—OPO₃H(alkyl)) and conjugate base group thereof (hereinafter, called an alkylphosphonatooxy group), a monoarylphosphonooxy group (—OPO₃H(aryl)) and conjugate base group thereof (hereinafter, called an arylphosphonatooxy group), a cyano group, and a nitro group. These substituents may be further substituted with the above-mentioned substituent, and may form a ring if this is possible.

R¹² to R¹⁷ independently denote a hydrogen atom, a halogen atom, or a monovalent organic group.

Examples of the halogen atom denoted by R¹² to R¹⁷ include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom, and a fluorine atom, a chlorine atom, or a bromine atom is preferable.

Examples of the monovalent organic group denoted by R¹² to R¹⁷ include a hydroxyl group, an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a cyclic hydrocarbon group, a heterocyclic group, an alkoxy group, an aryloxy group, an acyl group, an alkoxycarbonyl group, an acyloxy group, —SO₃—R^(a), —NR^(b)R_(c), a cyano group, —SiR^(d)R^(e)R^(f), —SOR⁹, —SO₂R^(g), and a nitro group; R^(a) denotes a hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group, an aryl group, an arylalkyl group, an alkali metal atom, or quaternary ammonium, R^(b), R^(c), and R^(g) independently denote an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a cyclic hydrocarbon group, or a heterocyclic group, and R^(d) to R^(f) independently denote an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a cyclic hydrocarbon group, a heterocyclic group, an alkoxy group, or an aryloxy group.

The alkyl group, alkenyl group, alkynyl group, aryl group, cyclic hydrocarbon group, heterocyclic group, alkoxy group, and aryloxy group denoted by R¹² to R¹⁷ have the same meanings as those denoted by R⁷ to R¹¹ above, and preferred ranges are also the same. Furthermore, these groups may have the above-mentioned substituent.

The acyl group and the alkoxycarbonyl group denoted by R¹² to R¹⁷ preferably have 1 to 30 carbons in the carbon chain side, and particularly preferably 1 to 12, and may be a straight chain and may have the above-mentioned substituent.

The acyloxy group denoted by R¹² to R¹⁷ preferably has 1 to 30 carbons, and particularly preferably 1 to 12 carbons, and may be a straight chain and may have the above-mentioned substituent.

R^(a) in —SO₃—Ra denoted by R¹² to R¹⁷ is preferably a hydrogen atom, the above-mentioned alkyl group, which may have a substituent, the above-mentioned aryl group, which may have a substituent, a lithium atom, a sodium atom, or a potassium atom.

The above-mentioned alkyl group, alkenyl group, alkynyl group, aryl group, cyclic hydrocarbon group, and heterocyclic group denoted by R^(b) and R^(c) in —NR^(b)R^(c) have the same meanings as those denoted by R⁷ to R¹¹ above, and preferred ranges are also the same. Furthermore, these groups may have the above-mentioned substituent.

The alkyl group, alkenyl group, alkynyl group, aryl group, cyclic hydrocarbon group, heterocyclic group, alkoxy group, and aryloxy group denoted by R^(d) to R^(f) in —SiR^(d)R^(e)R^(f) have the same meanings as those denoted by R⁷ to R¹¹ above, and preferred ranges are also the same. Furthermore, these groups may have the above-mentioned substituent.

The alkyl group, alkenyl group, alkynyl group, aryl group, cyclic hydrocarbon group, and heterocyclic group denoted by R^(g) in —SOR^(g) or —SO₂R^(g) have the same meanings as those denoted by R⁷ to R¹¹ above, and preferred ranges are also the same. Furthermore, these groups may have the above-mentioned substituent.

Specific examples of the countercation in the specific onium salt (a) include countercations having the structures denoted by Ca-1 to Ca-41 below.

With regard to the ink composition of the present invention, since the onium salt (a) contained therein has a counteranion having a volume of 1,000 Å³ or greater, the nucleophilicity of the counteranion is low, the inhibition of a cationic polymerization growth reaction by the counteranion is suppressed, the polymerization efficiency improves, and the curing sensitivity increases.

Furthermore, when the counteranion of the specific onium salt (a) contains an electron-withdrawing group in its structure, compared with a compound containing no electron-withdrawing group, the nucleophilicity of the counteranion becomes lower, and the curing sensitivity improves.

A method for synthesizing the specific onium salt is not particularly limited, but, for example, a counteranion having a volume of 1,000 Å³ may be synthesized by a method described in J. Am. Chem. Soc., Vol. 127, p. 8958-8959 (2005), etc. Furthermore, various types of specific onium salts and starting materials therefor may be synthesized by a known anionic exchange method described in, for example, JP-A-6-184170.

As the specific onium salt (a) that can be used in the present invention, an onium salt having in combination one anion selected from the group consisting of An-1 to An-12 above as a counteranion and one cation selected from the group consisting of Ca-1 to Ca-41 above as a countercation can be cited preferably.

(b) Cationically Polymerizable Compound

The cationically polymerizable compound used in the present invention is not particularly limited as long as it is a compound that undergoes a polymerization reaction by virtue of an acid generated by the photo-acid generator , which will be described later, and is cured, and various types of cationically polymerizable monomers known as photo-cationically polymerizable monomers may be used. Examples of the cationically polymerizable monomer include epoxy compounds, vinyl ether compounds, oxetane compounds described in JP-A-6-9714, JP-A-2001-31892, JP-A-2001-40068, JP-A-2001-55507, JP-A-2001-310938, JP-A-2001-310937, JP-A-2001 -220526, etc. As the cationically polymerizable compound, for example, a cationically polymerizable type photocuring resin is known, and in recent years cationically photopolymerizable type photocuring resins sensitized to a visible light wavelength region of 400 nm or longer have been disclosed in, for example, JP-A-6-43633 and JP-A-8-324137. As the radically polymerizable compound, photocuring materials employing photopolymerizable compositions described in, for example, JP-A-7-159983, JP-B-7-31399, JP-A-8-224982, and JP-A-10-863 are known.

Examples of the epoxy compounds include aromatic epoxides, alicyclic epoxides, and aliphatic epoxides, and examples of the aromatic epoxide include di- or polyglycidyl ethers produced by a reaction between epichlorohydrin and a polyhydric phenol having at least one aromatic nucleus or an alkylene oxide adduct thereof; specific examples include di- or polyglycidyl ethers of bisphenol A or an alkylene oxide adduct thereof, di- or polyglycidyl ethers of hydrogenated bisphenol A or an alkylene oxide adduct thereof, and novolac type epoxy resins. Examples of the alkylene oxide above include ethylene oxide and propylene oxide.

Examples of the alicyclic epoxides include cyclohexene oxide- and cyclopentene oxide-containing compounds obtained by epoxidizing a compound having at least one cycloalkene ring such as a cyclohexene ring or a cyclopentene ring with an appropriate oxidizing agent such as hydrogen peroxide or a peracid.

Examples of the aliphatic epoxides include di- or polyglycidyl ethers of an aliphatic polyhydric alcohol or an alkylene oxide adduct thereof, and representative examples thereof include diglycidyl ethers of an alkylene glycol such as the diglycidyl ether of ethylene glycol, the diglycidyl ether of propylene glycol, and the diglycidyl ether of 1,6-hexanediol, polyglycidyl ethers of a polyhydric alcohol such as the di- or triglycidyl ether of glycerol or an alkylene oxide adduct thereof, and diglycidyl ethers of a polyalkylene glycol such as the diglycidyl ether of polyethylene glycol or an alkylene oxide adduct thereof and the diglycidyl ether of polypropylene glycol or an alkylene oxide adduct thereof. Examples of the alkylene oxide above include ethylene oxide and propylene oxide.

Detailed examples of monofunctional and polyfunctional epoxy compounds that can be used in the present invention are now given.

Examples of monofunctional epoxy compounds used in the present invention include phenyl glycidyl ether, p-tert-butylphenyl glycidyl ether, butyl glycidyl ether, 2-ethylhexyl glycidyl ether, allyl glycidyl ether, 1,2-butylene oxide, 1,3-butadiene monooxide, 1,2-epoxydodecane, epichlorohydrin, 1,2-epoxydecane, styrene oxide, cyclohexene oxide, 3-methacryloyloxymethylcyclohexene oxide, 3-acryloyloxymethylcyclohexene oxide, and 3-vinylcyclohexene oxide.

Furthermore, examples of polyfunctional epoxy compounds include bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, bisphenol S diglycidyl ether, brominated bisphenol A diglycidyl ether, brominated bisphenol F diglycidyl ether, brominated bisphenol S diglycidyl ether, epoxy novolac resins, hydrogenated bisphenol A diglycidyl ether, hydrogenated bisphenol F diglycidyl ether, hydrogenated bisphenol S diglycidyl ether, 3,4-epoxycyclohexenylmethyl-3′,4′-epoxycyclohexenecarboxylate, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-meta-dioxane, bis(3,4-epoxycyclohexylmethyl) adipate, vinylcyclohexene dioxide, 4-vinylepoxycyclohexane, bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate, 3,4-epoxy-6-methylcyclohexenyl 3′,4′-epoxy-6′-methylcyclohexenecarboxylate, methylenebis(3,4-epoxycyclohexane), dicyclopentadiene diepoxide, the di(3,4-epoxycyclohexylmethyl) ether of ethylene glycol, ethylene bis(3,4-epoxycyclohexanecarboxylate), dioctyl epoxyhexahydrophthalate, di-2-ethylhexyl epoxyhexahydrophthalate, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, glycerol triglycidyl ether, trimethylolpropane triglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, 1,13-tetradecadiene dioxide, limonene dioxide, 1,2,7,8-diepoxyoctane, and 1,2,5,6-diepoxycyclooctane.

Among these epoxy compounds, the aromatic epoxides and the alicyclic epoxides are preferable from the viewpoint of excellent curing speed, and the alicyclic epoxides are particularly preferable.

Examples of the vinyl ether compounds include di- or tri-vinyl ether compounds such as ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, propylene glycol divinyl ether, dipropylene glycol divinyl ether, butanediol divinyl ether, hexanediol divinyl ether, cyclohexanedimethanol divinyl ether, and trimethylolpropane trivinyl ether, and monovinyl ether compounds such as ethyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, octadecyl vinyl ether, cyclohexyl vinyl ether, hydroxybutyl vinyl ether, 2-ethylhexyl vinyl ether, cyclohexanedimethanol monovinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, isopropenyl vinyl ether-O-propylene carbonate, dodecyl vinyl ether, and diethylene glycol monovinyl ether.

Detailed examples of monofunctional vinyl ethers and polyfunctional vinyl ethers are given below.

Specific examples of monofunctional vinyl ethers include methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, n-butyl vinyl ether, t-butyl vinyl ether, 2-ethylhexyl vinyl ether, n-nonyl vinyl ether, lauryl vinyl ether, cyclohexyl vinyl ether, cyclohexylmethyl vinyl ether, 4-methylcyclohexylmethyl vinyl ether, benzyl vinyl ether, dicyclopentenyl vinyl ether, 2-dicyclopentenoxyethyl vinyl ether, methoxyethyl vinyl ether, ethoxyethyl vinyl ether, butoxyethyl vinyl ether, methoxyethoxyethyl vinyl ether, ethoxyethoxyethyl vinyl ether, methoxypolyethylene glycol vinyl ether, tetrahydrofurfuryl vinyl ether, 2-hydroxyethyl vinyl ether, 2-hydroxypropyl vinyl ether, 4-hydroxybutyl vinyl ether, 4-hydroxymethylcyclohexylmethyl vinyl ether, diethylene glycol monovinyl ether, polyethylene glycol vinyl ether, chloroethyl vinyl ether, chlorobutyl vinyl ether, chloroethoxyethyl vinyl ether, phenylethyl vinyl ether, and phenoxypolyethylene glycol vinyl ether.

Furthermore, examples of polyfunctional vinyl ethers include divinyl ethers such as ethylene glycol divinyl ether, diethylene glycol divinyl ether, polyethylene glycol divinyl ether, propylene glycol divinyl ether, butylene glycol divinyl ether, hexanediol divinyl ether, bisphenol A alkylene oxide divinyl ether, and bisphenol F alkylene oxide divinyl ether; and polyfunctional vinyl ethers such as trimethylolethane trivinyl ether, trimethylolpropane trivinyl ether, ditrimethylolpropane tetravinyl ether, glycerol trivinyl ether, pentaerythritol tetravinyl ether, dipentaerythritol pentavinyl ether, dipentaerythritol hexavinyl ether, an ethylene oxide adduct of trimethylolpropane trivinyl ether, a propylene oxide adduct of trimethylolpropane trivinyl ether, an ethylene oxide adduct of ditrimethylolpropane tetravinyl ether, a propylene oxide adduct of ditrimethylolpropane tetravinyl ether, an ethylene oxide adduct of pentaerythritol tetravinyl ether, a propylene oxide adduct of pentaerythritol tetravinyl ether, an ethylene oxide adduct of dipentaerythritol hexavinyl ether, and a propylene oxide adduct of dipentaerythritol hexavinyl ether.

As the vinyl ether compound, the di- or tri-vinyl ether compounds are preferable from the viewpoint of curability, adhesion to a recording medium, surface hardness of the image formed, etc., and the divinyl ether compounds are particularly preferable.

The oxetane compound in the present invention means a compound having at least one oxetane ring, and may be selected freely from known oxetane compounds such as those described in JP-A-2001-220526, JP-A-2001-310937, and JP-A-2003-341217.

As the compound having an oxetane ring that can be used in the ink composition of the present invention, a compound having 1 to 4 oxetane rings in the structure is preferable. In accordance with use of such a compound, it becomes easy to maintain the viscosity of the ink composition in a range that gives good handling properties and, furthermore, the cured ink can be given high adhesion to the recording medium, which is preferable.

Examples of compounds having 1 to 2 oxetane rings in the molecule include compounds represented by Formulae (1) to (3) below.

R^(a1) denotes a hydrogen atom, an alkyl group having 1 to 6 carbons, a fluoroalkyl group having 1 to 6 carbons, an allyl group, an aryl group, a furyl group, or a thienyl group. When there are two R^(a1) in the molecule, they may be identical to or different from each other.

Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, and a butyl group, and preferred examples of the fluoroalkyl group include those obtained by substituting any of the hydrogen atoms of the above alkyl groups with a fluorine atom.

R^(a2) denotes a hydrogen atom, an alkyl group having 1 to 6 carbons, an alkenyl group having 2 to 6 carbons, a group having an aromatic ring, an alkylcarbonyl group having 2 to 6 carbons, an alkoxycarbonyl group having 2 to 6 carbons, or an N-alkylcarbamoyl group having 2 to 6 carbons. Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, and a butyl group, examples of the alkenyl group include a 1-propenyl group, a 2-propenyl group, a 2-methyl-1-propenyl group, a 2-methyl-2-propenyl group, a 1-butenyl group, a 2-butenyl group, and a 3-butenyl group, and examples of the group having an aromatic ring include a phenyl group, a benzyl group, a fluorobenzyl group, a methoxybenzyl group, and a phenoxyethyl group. Examples of the alkylcarbonyl group include an ethylcarbonyl group, a propylcarbonyl group, and a butylcarbonyl group, examples of the alkoxycarbonyl group include an ethoxycarbonyl group, a propoxycarbonyl group, and a butoxycarbonyl group, and examples of the N-alkylcarbamoyl group include an ethylcarbamoyl group, a propylcarbamoyl group, a butylcarbamoyl group, and a pentylcarbamoyl group. Furthermore, it is possible for R² to have a subsituent, and the examples of the substituent include alkyl group, having 1 to 6 carbons and fluorine atom

R^(a3) denotes a linear or branched alkylene group, a linear or branched poly(alkyleneoxy) group, a linear or branched unsaturated hydrocarbon group, a carbonyl group, a carbonyl group-containing alkylene group, a carboxyl group-containing alkylene group, a carbamoyl group-containing alkylene group, or a group shown below. Examples of the alkylene group include an ethylene group, a propylene group, and a butylene group, and examples of the poly(alkyleneoxy) group include a poly(ethyleneoxy) group and a poly(propyleneoxy) group. Examples of the unsaturated hydrocarbon group include a propenylene group, a methylpropenylene group, and a butenylene group.

When R^(a3) is the above-mentioned polyvalent group, R^(a4) denotes a hydrogen atom, an alkyl group having 1 to 4 carbons, an alkoxy group having 1 to 4 carbons, a halogen atom, a nitro group, a cyano group, a mercapto group, a lower alkylcarboxyl group, a carboxyl group, or a carbamoyl group.

R^(a5) denotes an oxygen atom, a sulfur atom, a methylene group, NH, SO, SO₂, C(CF₃)₂, or, C(CH₃)₂.

R^(a6) denotes an alkyl group having 1 to 4 carbons or an aryl group, and n is an integer of 0 to 2,000. R^(a7) denotes an alkyl group having 1 to 4 carbons, an aryl group, or a monovalent group having the structure below. In the formula, Ra8 denotes an alkyl group having 1 to 4 carbons or an aryl group, and m is an integer of 0 to 100.

Examples of the compound represented by Formula (1) include 3-ethyl-3-hydroxymethyloxetane (OXT-101: manufactured by Toagosei Co., Ltd.), 3-ethyl-3-(2-ethylhexyloxymethyl)oxetane (OXT-212: manufactured by Toagosei Co., Ltd.), and 3-ethyl-3-phenoxymethyloxetane (OXT-211: manufactured by Toagosei Co., Ltd.). Examples of the compound represented by Formula (2) include 1,4-bis[(3-ethyl-3-oxetanylmethoxy)methyl]benzene (OXT-121: Toagosei Co., Ltd.). Examples of the compound represented by Formula (3) include bis(3-ethyl-3-oxetanylmethyl) ether (OXT-221: Toagosei Co., Ltd.).

Examples of the compound having 3 to 4 oxetane rings in the molecule include compounds represented by Formula (4) below.

In Formula (4), R^(a1) denotes the same as in Formula (1) above. Furthermore, examples of R^(a9), which is a polyvalent linking group, include a branched alkylene group having 1 to 12 carbons such as a group represented by A to C below, a branched poly(alkyleneoxy) group such as a group represented by D below, and a branched polysiloxane group such as a group represented by E below. j is 3 or 4.

In the above A, R^(a10) denotes a methyl group, an ethyl group, or a propyl group. Furthermore, in the above D, p is an integer of 1 to 10.

Moreover, as another embodiment of the oxetane compound that can be suitably used in the present invention, a compound having an oxetane ring on a side chain, represented by Formula (5) below, can be cited.

In Formula (5), R^(a1) and R^(a8) denote the same as in the above-mentioned formulae. R^(a11) is an alkyl group having 1 to 4 carbons such as a methyl group, an ethyl group, a propyl group, or a butyl group, or a trialkylsilyl group, and r is 1 to 4.

Such compounds having an oxetane ring are described in detail in paragraph Nos. (0021) to (0084) of JP-A-2003-341217 above, and the compounds described here may be suitably used in the present invention.

The oxetane compounds described in JP-A-2004-91556 can be used in the present invention. The details are described in paragraph Nos. (0022) to (0058).

Among the oxetane compounds used in the present invention, from the viewpoint of ink composition viscosity and tackiness, it is preferable to use a compound having one oxetane ring.

The ink composition of the present invention may comprise only one type of cationically polymerizable compound or two or more types thereof in combination, but from the viewpoint of suppressing effectively shrinkage during ink curing, it is preferable to use a combination of a vinyl ether compound and at least one type of compound selected from the oxetane compounds and the epoxy compounds.

The content of the cationically polymerizable compound in the ink composition is suitably in the range of 10 to 95 wt % relative to the total solids content of the ink composition, preferably 30 to 90 wt %, and more preferably 50 to 85 wt %.

(c) Colorant

The colorant that can be used in the present invention is not particularly limited, but a pigment and an oil-soluble dye that have excellent weather resistance and rich color reproduction are preferable, and it may be selected from any known colorant such as a soluble dye. It is preferable that the colorant that can be suitably used in the ink composition or the inkjet recording ink composition of the present invention does not function as a polymerization inhibitor in a polymerization reaction, which is a curing reaction. This is because the sensitivity of the curing reaction by actinic radiation should not be degraded.

(c-1) Pigment

The pigment that can be used in the present invention is not particularly limited and, for example, organic and inorganic pigments having the numbers below described in the Color Index may be used.

That is, as a red or magenta pigment, Pigment Red 3, 5, 19, 22, 31, 38, 43, 48:1, 48:2, 48:3, 48:4, 48:5, 49:1, 53:1, 57:1, 57:2, 58:4, 63:1, 81, 81:1, 81:2, 81:3, 81:4, 88, 104, 108, 112, 122, 123, 144, 146, 149, 166, 168, 169, 170, 177, 178, 179, 184, 185, 208, 216, 226, or 257, Pigment Violet 3, 19, 23, 29, 30, 37, 50, or 88, and Pigment Orange 13, 16, 20, or 36; as a blue or cyan pigment, Pigment Blue 1, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 17-1, 22, 27, 28, 29, 36, or 60; as a green pigment, Pigment Green 7, 26, 36, or 50; as a yellow pigment, Pigment Yellow 1, 3, 12, 13, 14, 17, 34, 35, 37, 55, 74, 81, 83, 93, 94, 95, 97, 108, 109, 110, 137, 138, 139, 153, 154, 155, 157, 166, 167, 168, 180, 185, or 193; as a black pigment, Pigment Black 7, 28, or 26; as a white pigment, Pigment White 6, 18, or 21, etc. may be used according to the intended application.

(c-2) Oil-Soluble Dye

The oil-soluble dye that can be used in the present invention is explained below.

The oil-soluble dye that can be used in the present invention means a dye that is substantially insoluble in water. Specifically, the solubility in water at 25° C (the mass of dye that can be dissolved in 100 g of water) is no greater than 1 g, preferably no greater than 0.5 g, and more preferably no greater than 0.1 g. Therefore, the oil-soluble dye means a so-called water-insoluble pigment or an oil-soluble dye, and among these the oil-soluble dye is preferable.

In the present invention, the oil-soluble dye may be used singly or in a combination of two or more types. Furthermore, another colorant such as a water-soluble dye, a disperse dye, or a pigment may be contained as necessary in a range that does not interfere with the effects of the present invention.

Among the oil-soluble dyes that can be used in the present invention, as a yellow dye, any may be used. Examples thereof include aryl or heteryl azo dyes having a coupling component such as a phenol, a naphthol, an aniline, a pyrazolone, a pyridone, or an open-chain active methylene compound; azomethine dyes having a coupling component such as an open-chain active methylene compound; methine dyes such as benzylidene dyes and monomethineoxonol dyes; quinone dyes such as naphthoquinone dyes and anthraquinone dyes; and other dye species such as quinophthalone dyes, nitro/nitroso dyes, acridine dyes, and acridinone dyes.

Among the above-mentioned oil-soluble dyes that can be used in the present invention, as a magenta dye, any may be used. Examples thereof include aryl or heteryl azo dyes having a coupling component such as a phenol, a naphthol, or an aniline; azomethine dyes having a coupling component such as a pyrazolone or a pyrazolotriazole; methine dyes such as arylidene dyes, styryl dyes, merocyanine dyes, and oxonol dyes; carbonium dyes such as diphenylmethane dyes, triphenylmethane dyes, and xanthene dyes; quinone dyes such as naphthoquinones, anthraquinones, or anthrapyridones; and condensed polycyclic dyes such as dioxazine dyes.

Among the oil-soluble dyes that can be used in the present invention, as a cyan dye, any may be used. Examples thereof include indoaniline dyes, indophenol dyes, and azomethine dyes having a coupling component such as a pyrrolotriazole; polymethine dyes such as cyanine dyes, oxonol dyes, and merocyanine dyes; carbonium dyes such as diphenylmethane dyes, triphenylmethane dyes, and xanthene dyes; phthalocyanine dyes; anthraquinone dyes; aryl or heteryl azo dyes having a coupling component such as a phenol, a naphthol, or an aniline; and indigo/thioindigo dyes.

The above-mentioned dyes may be dyes that exhibit respective colors of yellow, magenta, and cyan only after a part of the chromophore dissociates, and in that case the counter cation may be an inorganic cation such as an alkali metal or ammonium, may be an organic cation such as pyridinium or a quaternary ammonium salt, or may be a polymer cation having the above cation as a partial structure.

Although not limited to the following, preferred specific examples thereof include CI Solvent Black 3, 7, 27, 29, and 34; CI Solvent Yellow 14, 16, 19, 29, 30, 56, 82, 93, and 162; CI Solvent Red 1, 3, 8, 18, 24, 27, 43, 49, 51, 72, 73, 109, 122, 132, and 218; CI Solvent Violet 3; CI Solvent Blue 2, 11, 25, 35, 38, 67, and 70; CI Solvent Green 3 and 7; and CI Solvent Orange 2. Particularly preferred examples thereof include Nubian Black PC-0850, Oil Black HBB, Oil Yellow 129, Oil Yellow 105, Oil Pink 312, Oil Red 5B, Oil Scarlet 308, Vali Fast Blue 2606, Oil Blue BOS (manufactured by Orient Chemical Industries, Ltd.), Aizen Spilon Blue GNH (manufactured by Hodogaya Chemical Co., Ltd.), Neopen Yellow 075, Neopen Magenta SE1378, Neopen Blue 808, Neopen Blue FF4012, and Neopen Cyan FF4238 (manufactured by BASF).

In the present invention, a disperse dye may be used in a range that enables it to be dissolved in a water-immiscible organic solvent. Specific preferred examples thereof include CI Disperse Yellow 5, 42, 54, 64, 79, 82, 83, 93, 99, 100, 119, 122, 124, 126, 160, 184:1, 186, 198, 199, 201, 204, 224, and 237; CI Disperse Orange 13, 29, 31:1, 33, 49, 54, 55, 66, 73, 118, 119, and 163; CI Disperse Red 54, 60, 72, 73, 86, 88, 91, 92, 93, 111, 126, 127, 134, 135, 143, 145, 152, 153, 154, 159, 164, 167:1, 177, 181, 204, 206, 207, 221, 239, 240, 258, 277, 278, 283, 311, 323, 343, 348, 356, and 362; CI Disperse Violet 33; CI Disperse Blue 56, 60, 73, 87, 113, 128, 143, 148, 154, 158, 165, 165:1, 165:2, 176, 183, 185, 197, 198, 201, 214, 224, 225, 257, 266, 267, 287, 354, 358, 365, and 368; and CI Disperse Green 6:1 and 9.

Particularly preferred examples of the oil-soluble dye include azo and azomethine dyes represented by Formulae (1) and (2) below. Dyes represented by Formula (2) below are known, in the photographic material area, as dyes that are generated from a coupler and a developing agent by oxidation.

In Formulae (1) and (2) above, R¹, R², R³ and R⁴ independently denote a hydrogen atom, a halogen atom, an aliphatic group, an aromatic group, a heterocyclic group, a cyano group, a hydroxyl group, a nitro group, an amino group, an alkylamino group, an alkoxy group, an aryloxy group, an amide group, an arylamino group, a ureido group, a sulfamoylamino group, an alkylthio group, an arylthio group, an alkoxycarbonylamino group, a sulfonamide group, a carbamoyl group, a sulfamoyl group, a sulfonyl group, an alkoxycarbonyl group, a heterocyclooxy group, an azo group, an acyloxy group, a carbamoyloxy group, a silyloxy group, an aryloxycarbonyl group, an aryloxycarbonylamino group, an imide group, a heterocyclothio group, a sulfinyl group, a phosphoryl group, an acyl group, a carboxyl group, or a sulfo group.

In Formulae (1) and (2) above, in particular, R² is preferably, among the above-mentioned substituents, a hydrogen atom, a halogen atom, an aliphatic group, an alkoxy group, an aryloxy group, an amide group, a ureido group, a sulfamoylamino group, an alkoxycarbonylamino, or a sulfonamide group.

In the present specification, the aliphatic group denotes an alkyl group, a substituted alkyl group, an alkenyl group, a substituted alkenyl group, an alkynyl group, a substituted alkynyl group, an aralkyl group, or a substituted aralkyl group. The aliphatic group may have a branch or form a ring. The number of carbon atoms of the aliphatic group is preferably 1 to 20, and more preferably 1 to 18. The aryl moiety of the aralkyl group and the substituted aralkyl group is preferably phenyl or naphthyl, and particularly preferably phenyl. Examples of the substituents of the alkyl moieties of the substituted alkyl group, the substituted alkenyl group, the substituted alkynyl group, and the substituted aralkyl group include the substituents cited for explanation of R¹ to R⁴. Examples of the substituents of the aryl moiety of the substituted aralkyl group are the same as those of the substituent of the substituted aryl group below.

In the present specification, the aromatic group means an aryl group and a substituted aryl group. The aryl group is preferably phenyl or naphthyl, and particularly preferably phenyl. The aryl moiety of the substituted aryl group is the same as that of the above-mentioned aryl group. Examples of the substituent of the substituted aryl group include substituents cited for explanation of R¹ to R⁴.

In Formulae (1) and (2) above, A denotes —NR⁵R⁶ or a hydroxyl group, and R⁵ and R⁶ independently denote a hydrogen atom, an aliphatic group, an aromatic group, or a heterocyclic group. A is preferably —NR⁵R . R⁵ and R⁶ may be bonded together to form a ring. R⁵ and R⁶ preferably each denote a hydrogen atom, an alkyl group, a substituted alkyl group, an aryl group, or a substituted aryl group, and most preferably a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, or a substituted alkyl group having 1 to 18 carbon atoms.

In Formula (2) above, B¹ denotes ═C(R³)— or ═N—, and B² denotes —C(R⁴)═ or —N═. It is preferable that B¹ and B² are not —N═ at the same time, and it is more preferable that B¹ is ═C(R³)— and B² is —C(R⁴)═. Any of R¹ and R⁵, R³ and R⁶, and R¹ and R² may be bonded together to form an aromatic ring or a hetero ring.

In Formula (1) above, Y denotes an unsaturated heterocyclic group. Y is preferably a five-membered or six-membered unsaturated hetero ring. The hetero ring may be condensed with an aliphatic ring, an aromatic ring, or another hetero ring. Examples of the hetero atom of the hetero ring include N, O, and S.

Preferred examples of the above-mentioned unsaturated hetero ring include a pyrazole ring, an imidazole ring, a thiazole ring, an isothiazole ring, a thiadiazole ring, a thiophene ring, a benzothiazole ring, a benzoxazole ring, a benzoisothiazole ring, a pyrimidine ring, a pyridine ring, and a quinoline ring. It is also possible for the unsaturated heterocyclic group to have a substituent cited above as R¹ to R⁴.

In Formula (2) above, X denotes a color photographic coupler residue. Preferred examples of the color photographic coupler residue are as follows.

Yellow couplers: U.S. Pat. Nos. 3,933,501, 4,022,620, 4,326,024, and 4,401,752, couplers represented by Formulae (I) and (II) in U.S. Pat. No.4,248,961, JP-B-58-10739, GB Pat. Nos. 1,425,020 and 1,476,760, U.S. Pat. Nos. 3,973,968, 4,314,023, and 4,511,649, and EP Pat. Nos. 249,473A and 502,424A; couplers represented by Formulae (1) and (2) in EP Pat. No. 513,496A (in particular, Y-28 on page 18); couplers represented by Formula (I) of claim 1 in EP Pat. No. 568,037A; couplers represented by Formula (I) of lines 45 to 55 in Column 1 in U.S. Pat. No. 5,066,576; couplers represented by Formula (I) in Paragraph 0008 in JP-A-4-274425; couplers of claim 1 on page 40 in EP Pat. No. 498,381A1 (in particular, D-35 on page 18); couplers represented by Formula (Y) on page 4 in EP Pat. No. 447,969A1 (in particular, Y-1 (page 17) and Y-54 (page 41)); and couplers represented by Formulae (II) to (IV) on lines 36 to 58 of Column 7 in U.S. Pat. No. 4,476,219 (in particular, 11-17 and 19 (Column 17), and 11-24 (Column 1 9)).

Magenta couplers: U.S. Pat. Nos. 4,310,619 and 4,351,897, EP Pat. No. 73,636, U.S. Pat. Nos. 3,061,432 and 3,725,067, Research Disclosure No. 24220 (June, 1984) and No. 24230 (June, 1984), JP-A-60-33552, JP-A-60-43659, JP-A-61-72238, JP-A-60-35730, JP-A-55-118034, JP-A-60-185951, U.S. Pat. Nos. 4,500,630, 4,540,654, and 4,556,630, WO88/04795, JP-A-3-39737 (L-57 (page 11, lower right), L-68 (page 12, lower right), L-77 (page 13, lower right)), EP Pat. No. 456,257 [A-4]-63 (p. 134), [A-4]-73, -75 (p. 139), EP Pat. No. 486,965 M-4, -6 (p. 26), M-7 (p. 27), EP Pat. No. 571,959A M-45 (p. 19), JP-A-5-204106 M-1 (p. 6), and JP-A-4-362631 paragraph No. 0237, M-22, and U.S. Pat. Nos. 3,061,432 and 3,725,067.

Furthermore, couplers described in JP-A-62-215272 (p. 91), JP-A-2-33144 (p. 3 and 30), EP 355,660A (p. 4, 5, 45 and 47) are also useful.

Cyan coupler: U.S. Pat. Nos. 4,052,212, 4,146,396, 4,228,233, and 4,296,200, EP Pat. No. 73,636, JP-A-4-204843, CX-1, 3, 4, 5, 11, 12, 14, 15 (pp. 14 to 16); JP-A-4-43345 C-7, 10 (p. 35), 34, 35 (p. 37), (1-1), (1-17) (pp. 42 to 43); and couplers represented by Formula (la) or (lb) of claim 1 in JP-A-6-67385.

Among the oil-soluble dyes represented by Formula (1) above, the magenta dyes particularly preferably used are dyes represented by Formula (3) below.

In Formula (3) above, Z¹ denotes an electron-attracting group having a Hammett substituent constant σp value of equal to or greater than 0.20. Z¹ is preferably an electron-attracting group having a σp value of at least 0.30 but no greater than 1.0. Preferred specific examples of the substituent include electron-attracting substituents that are described later, and among them an acyl group having 2 to 12 carbons, an alkyloxy carbonyl group having 2 to 12 carbons, a nitro group, a cyano group, an alkylsulfonyl group having 1 to 12 carbons, an arylsulfonyl group having 6 to 18 carbons, a carbamoyl group having 1 to 12 carbons, and a haloalkyl group having 1 to 12 carbons are preferable. A cyano group, an alkylsulfonyl group having 1 to 12 carbons, and an arylsulfonyl group having 6 to 18 carbons are particularly preferable, and a cyano group is most preferable.

In Formula (3) above, Z² denotes a hydrogen atom, an aliphatic group, or an aromatic group.

In Formula (3) above, R¹ to R⁶ are the same as the corresponding ones of Formula (1) above.

In Formula (3) above, Q denotes a hydrogen atom, an aliphatic group, an aromatic group, or a heterocyclic group. Among them, Q is preferably a group formed from a group of non-metal atoms necessary to form a 5- to 8-membered ring. Among them an aromatic group and a heterocyclic group are particularly preferable. The 5- to 8-membered ring may be substituted, may be a saturated ring, or may have an unsaturated bond. Preferred examples of the non-metal atom include a nitrogen atom, an oxygen atom, a sulfur atom, and a carbon atom. Specific examples of such ring structures include a benzene ring, a cyclopentane ring, a cyclohexane ring, a cycloheptane ring, a cyclooctane ring, a cyclohexene ring, a pyridine ring, a pyrimidine ring, a pyrazine ring, a pyridazine ring, a triazine ring, an imidazole ring, a benzoimidazole ring, an oxazole ring, a benzoxazole ring, an oxane ring, a sulfolane ring, and a thiane ring, and in a case where these rings have a further substituent, examples of the substituent include groups cited as examples of substituents R¹ to R⁴ in Formula (1) above.

Preferred structures of the compounds represented by Formula (3) above are described in JP-A-2001-335714.

Among the dyes represented by Formula (2) above, the magenta dye particularly preferably employs a dye represented by Formula (4) below.

In Formula (4) above, G denotes a hydrogen atom, an aliphatic group, an aromatic group, a heterocyclic group, a cyano group, an alkoxy group, an aryloxy group, an alkylthio group, an arylthio group, an ester group, an amino group, a carbamoyl group, a sulfonyl group, a sulfamoyl group, a ureido group, a urethane group, an acyl group, an amide group, or a sulfonamide group.

In Formula (4) above, R¹, R², A, B¹, and B² are the same as the corresponding ones of Formula (2) above, and preferred ranges are also the same.

In Formula (4) above, L denotes an atomic group forming a five-membered or six-membered nitrogen-containing hetero ring, which may be substituted with at least one of an aliphatic group, an aromatic group, a heterocyclic group, a cyano group, an alkoxy group, an aryloxy group, an alkylthio group, an arylthio group, an ester group, an amino group, a carbamoyl group, a sulfonyl group, a sulfamoyl group, a ureido group, a urethane group, an acyl group, an amide group, and a sulfonamide group, and this hetero ring may further form a condensed ring with another ring.

With regard to compounds represented by Formula (4) above, A is preferably —NR⁵R⁶, and L.preferably forms a five-membered nitrogen-containing hetero ring; examples of the five-membered nitrogen-containing hetero ring include an imidazole ring, a triazole ring, and a tetrazole ring.

Among the dyes represented by Formula (1) and Formula (2) above, compound examples (M0, M-1 to 6, a-21 to 25) for a magenta dye are shown below, but these are only for explaining the present invention in detail, and the present invention should not be construed as being limited thereto.

In the present invention, M0, M-4, M-6, or a-21 may be used, and M-4, M-6, and a-21 are particularly preferable.

TABLE 1

R₁ a-21

a-22

a-23

a-24

a-25

R₂ a-21

a-22

a-23

a-24

a-25

R₃ R₄ a-21

a-22

a-23

a-24

a-25

C₈H₁₇(t)

Other compound examples of the colorant that can be used in the present invention include those described in JP-A-2001-240763, 2001-181549, and JP-A-2001-335714, but the present invention should not be construed as being limited thereto.

The compound represented by Formula (3) above may be synthesized by reference to a method described in, for example, JP-A-2001-335714 or JP-A-55-161856. The compound represented by Formula (4) above may be synthesized by reference to a method described in, for example, JP-A-4-126772, JP-B-7-94180, or JP-A-2001-240763.

Among the dyes represented by Formula (2) above, as a cyan dye a pyrrolotriazole azomethine dye represented by Formula (5) below is particularly preferably used.

In Formula (5) above, A, R¹, R², B¹, and B² are the same as the corresponding ones of Formula (2) above, and preferred ranges thereof are also the same.

In Formula (5) above, Z³ and Z⁴ are independently the same as G in Formula (4) above. Z³ and Z⁴ may be bonded together to form a ring structure. One in which Z³ is an electron-attracting group having a Hammett substituent constant up value of equal to or greater than 0.30 exhibits a sharp absorption and is more preferable. Moreover, one in which Z³ is an electron-attracting group having a Hammett substituent constant up value of equal to or greater than 0.45 is more preferable, and an electron-attracting group having a Hammett substituent constant σp value of equal to or greater than 0.60 is most preferable. Furthermore, one in which the sum of the Hammett substituent constant up values of Z³ and Z⁴ is equal to or greater than 0.70 exhibits excellent hue of a cyancolor, and is more preferable.

In Formula (5) above, M is an atomic group forming a 1,2,4-triazole ring that is condensed with the 5-membered ring of Formula (5) above; either one of the two atoms B³ and B⁴ at the sites of condensation with the 5-membered ring is a nitrogen atom, and the other is a carbon atom.

The compound represented by Formula (5) above is preferably used as a cyan dye, but it may be used as a magenta dye by changing a substituent.

The Hammett substituent constant up value used in the present specification is now- explained. The Hammett rule is an empirical rule proposed by L. P. Hammett in 1935 in order to quantitatively deal with the influence of a substituent on a reaction or equilibrium of a benzene derivative, and the validity thereof is currently widely recognized. A σp value and a σm value are required for the substituent constant in the Hammett rule, and details of these values can be referred to in many general books, for example, ‘Lange's Handbook of Chemistry’, Ed. by J. A. Dean, 12th edition, 1979 (Mc Graw-Hill) or ‘Kagakuno Ryouiki’ (Journal of Japanese Chemistry), special issue, 122, pp. 96 to 103, 1979 (Nankodo Co., Ltd.). In the present invention, the substituents are limited or explained using the Hammett substituent constant σp, but this does not mean that they are limited to substituents whose values are described in published references, and a substituent whose value is not published in the references but is included in the range if it is measured in accordance with the Hammett rule is of course included. Among Formulae (1) to (5) above, those that are not benzene derivatives are also included, but the ap value is used as a scale showing the electronic effect of the substituent, irrespective of the position of substitution. The σp value in the present invention is used with the above-mentioned meaning.

Examples of electron-attracting groups having a Hammett substituent constant σp value of equal to or greater than 0.60 include a cyano group, a nitro group, an alkylsulfonyl group (e.g. a methanesulfonyl group), and an aryisulfonyl group (e.g. a benzenesulfonyl group). Examples of electron-attracting groups having a Hammett ap value of equal to or greater than 0.45 include, in addition to the above, an acyl group (e.g. an acetyl group), an alkoxycarbonyl group (e.g. a dodecyloxycarbonyl group), an aryloxycarbonyl group (e.g. m-chlorophenoxycarbonyl), an alkylsulfinyl group (e.g. n-propylsulfinyl), an arylsulfinyl group (e.g. phenylsulfinyl), a sulfamoyl group (e.g. N-ethylsulfamoyl, N,N-dimethylsulfamoyl), and a haloalkyl group (e.g. trifluoromethyl).

Examples of electron-attracting groups having a Hammett substituent constant ap value of equal to or greater than 0.30 include, in addition to the above, an acyloxy group (e.g. acetoxy), a carbamoyl group (e.g. N-ethylcarbamoyl, N,N-dibutylcarbamoyl), a haloalkoxy group (e.g. trifluoromethyloxy), a haloaryloxy group (e.g. pentafluorophenyloxy), a sulfonyloxy group (e.g. a methylsulfonyloxy group), a haloalkylthio group (e.g. difluoromethylthio), an aryl group substituted with two or more electron-attracting groups having a σp value of equal to or greater than 0.15 (e.g. 2,4-dinitrophenyl, pentachlorophenyl), and a hetero ring (e.g. 2-benzooxazolyl, 2-benzothiazolyl, 1-phenyl-2-benzimidazolyl). Specific examples of electron-attracting groups having a ap value of equal to or greater than 0.20 include, in addition to the above, a halogen atom.

Furthermore, in the present invention, an oil-soluble dye represented by Formula (A-I) below can be used preferably.

In Formula (A-I): X₁, X₂, X₃, and X₄ independently denote a group selected from —SO-Z, 13 SO₂-Z, —SO₂NR₁R₂, —CONR₁R₂, —CO₂R₁, and a sulfo group. Here, Z denotes a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group. R₁ and R₂ independently denote a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, provided that R₁ and R₂ are not both hydrogen atoms. M denotes a hydrogen atom, a metal element, a metal oxide, a metal hydroxide, or a metal halide. Y₁, Y₂, Y₃, and Y₄ independently denote a hydrogen atom or a monovalent substituent. a₁ to a₄ and b₁ to b₄ denote the numbers of X₁ to X₄ and Y₁ to Y₄, and independently denote an integer of 0 to 4, provided that the sum total of a₁ to a₄ is equal to or greater than 2.

Among the oil-soluble dyes represented by Formula (A-I) above, an oil-soluble dye represented by Formula (A-II) below may particularly preferably be used.

In Formula (A-II): X₁₁ to X₁₄, Y₁₁ to Y₁₈, and M are the same as X₁ to X₄, Y₁ to Y₄, and M in Formula (A-I) respectively. a₁₁ to a₁₄ independently denote an integer of 1 or 2.

As a specific example of Formula (A-II) above, a compound example (AII-17) is cited, but this is for explaining the present invention in detail, and the present invention should not be construed as being limited thereto.

Compound No. M X a AII-17 Cu

1 AII-18 Cu

1 AII-19 Cu

1 AII-20 Cu

1 AII-21 Cu

1 AII-22 Cu

1 AII-23 Cu

1

In the present invention, it is preferable to use an oil-soluble dye having an oxidation potential that is more noble than 1.0 V (SCE). The more noble the oxidation potential, the more preferable it is; it is more preferable to use one having an oxidation potential more noble than 1.1 V (SCE), and it is most preferable to use one having an oxidation potential more noble than 1.2 V (SCE).

The oxidation potential value (Eox) can be easily measured by one skilled in the art and a method therefor is described in, for example, P. Delahay, ‘New Instrumental Methods in Electrochemistry’, Interscience Publishers (1954), A. J. Bard et al., ‘Electrochemical Methods’, John Wiley & Sons (1980), and Akira Fujishima et al., ‘Denkikagaku Sokuteihou’ (Electrochemical Measurement Methods), Gihodo Shuppan Sha (1984).

More specifically, a test sample is dissolved to give a concentration of 1×10⁻⁴ to 1×10⁻⁶ mol/L in a solvent such as dimethylformamide or acetonitrile containing a supporting electrolyte such as sodium perchlorate or tetrapropylammonium perchlorate, an oxidation wave when sweeping toward the oxidation side (noble side) using carbon (GC) as a working electrode and a rotating platinum electrode as the counter electrode using cyclic voltammetry or direct current polarographic equipment is approximated to a straight line, and the oxidation potential of the midpoint of a line segment formed between an intersection point of the straight line and a residual current/potential straight line and an intersection point of the straight line and a saturated current straight line (or an intersection point with a straight line parallel to the ordinate passing through the potential peak value) is measured as a value relative to the SCE (saturated calomel electrode). This value sometimes deviates by on the order of tens of millivolts due to the effect of a liquid junction potential, the liquid resistance of the sample solution, or the like, but the reproducibility of the potential can be guaranteed by adding a standard sample (for example, hydroquinone). The support electrolyte and solvent used may be selected appropriately according to the oxidation potential and the solubility of the test sample. The support electrolyte and solvent that can be used here may be referred to in Akira Fujishima et al., ‘Denkikagaku Sokuteihou’ (Electrochemical Measurement Methods), Gihodo Shuppan Sha (1984), pp.101 to 118.

In the concentration range of the above-mentioned measurement solvent and a phthalocyanine compound sample, the oxidation potential of a disassociated state is measured.

The value of Eox represents the ease of electron transfer from a sample to an electrode; the larger the value (the more noble the oxidation potential), the more difficult it is for electrons to transfer from the sample to the electrode, in other words, the more difficult it is to oxidize.

If a dye having a low oxidation potential is used, polymerization is greatly inhibited by the dye, and the curability is degraded. When a dye having a noble oxidation potential is used, there is hardly any inhibition of polymerization.

The colorant that can be used in the present invention is preferably added to the ink composition or the inkjet recording ink composition of the present invention and then dispersed in the ink to an appropriate degree. For dispersion of the colorant, for example, a dispersing machine such as a ball mill, a sand mill, an attritor, a roll mill, an agitator, a Henschel mixer, a colloidal mill, an ultrasonic homogenizer, a pearl mill, a wet type jet mill, or a paint shaker may be used.

When carrying out dispersion of the colorant, a dispersant may be added. The type of dispersant is not particularly limited, but it is preferable to use a polymeric dispersant, and examples of the polymeric dispersant include the Solsperse series manufactured by Avecia. Furthermore, as a dispersion adjuvant, it is also possible to use a synergist, according to the various types of pigment. In the present invention, the dispersant and dispersion adjuvant are preferably added at 1 to 50 parts by weight relative to 100 parts by weight of the pigment.

The colorant may be added directly to the ink composition of the present invention, but in order to improve dispersibility it may be added in advance to a solvent or a dispersing medium such as a polymerizable compound used in the present invention. In the present invention, in order to avoid the problem of the solvent resistance being degraded when the solvent remains in the cured image and the VOC (Volatile Organic Compound) problem of the residual solvent, it is preferable to add the colorant to a polymerizable compound. As a polymerizable compound used, it is preferable in terms of dispersion suitability to select a monomer having the lowest viscosity.

In the present invention, it is preferable for the average particle size of the colorant to be in the range of 0.005 to 0.5 μm, more preferably 0.01 to 0.45 μm, and yet more preferably, 0.015 to 0.3 μm. Furthermore, it is preferable for the maximum particle size of the colorant to be 0.3 to 10 μm, and more preferably 0.3 to 3 μm. It is preferable, in order to make the maximum particle size of the pigment particles be in the above-mentioned range, that the colorant, the dispersant, and the dispersing medium are selected, and dispersion conditions and filtration conditions are set. By such control of particle size, clogging of a head nozzle can be suppressed, and the ink storage stability, the ink transparency, and the curing sensitivity can be maintained, which is preferable.

(d) Sensitizing Dye

The ink composition of the present invention may contain (d) a sensitizing dye in order to promote decomposition of the above-mentioned polymerization initiator by absorbing specific actinic radiation, in particular when used for inkjet recording. The sensitizing dye absorbs specific actinic radiation and attains an electronically excited state. The sensitizing dye in the electronically excited state causes actions such as electron transfer, energy transfer, or heat generation upon contact with the polymerization initiator. This causes the polymerization initiator to undergo a chemical change and decompose, thus forming a radical, an acid, or a base.

Preferred examples of the sensitizing dye include those in the categories of compounds below and have an adsorption wavelength in the region of 350 nm to 450 nm.

Examples thereof include polynuclear aromatic compounds (e.g. pyrene, perylene, triphenylene), xanthenes (e.g. fluorescein, eosin, erythrosine, rhodamine B, rose bengal), cyanines (e.g. thiacarbocyanine, oxacarbocyanine), merocyanines (e.g. merocyanine, carbomerocyanine), thiazines (e.g. thionine, methylene blue, toluidine blue), acridines (e.g. acridine orange, chloroflavine, acriflavine), anthraquinones (e.g. anthraquinone), squaryliums (e.g. squarylium), and coumarins (e.g. 7-diethylamino-4-methylcoumarin).

Preferred examples of the sensitizing dye that can be used in the present invention include compounds represented by Formulae (vi) to (x) below.

In Formula (vi), A¹ denotes a sulfur atom or NR⁵⁰, and R⁵⁰ denotes an alkyl group or an aryl group. L¹ denotes a non-metallic atomic group forming a basic nucleus of a dye in cooperation with the adjacent A¹ and adjacent carbon atom. R⁵¹ and R⁵² independently denote a hydrogen atom or a monovalent non-metallic atomic group, and R⁵¹ and R⁵² may be bonded together to form an acidic nucleus of a dye. W denotes an oxygen atom or a sulfur atom.

In Formula (vii), Ar¹ and Ar² independently denote an aryl group and are connected to each other via bonding with L². Here, L² denotes —O— or —S—. W has the same meaning as that shown in Formula (vi).

In Formula (viii), A² denotes a sulfur atom or NR⁵⁹, and R⁵⁹ denotes an alkyl group or an aryl group. L³ denotes a non-metallic atomic group forming a basic nucleus of a dye in cooperation with the adjacent A² and carbon atom. R⁵³, R⁵⁴, R⁵⁵, R⁵⁶, R⁵⁷, and R⁵⁸ independently denote a monovalent non-metallic atomic group.

In Formula (ix), A³ and A⁴ independently denote —S—, —NR⁶²—, or —NR⁶³—, and R⁶² and R⁶³ independently denote a substituted or unsubstituted alkyl group or a substituted or unsubstituted aryl group. L⁴ and L⁵ independently denote a non-metallic atomic group forming a basic nucleus of a dye in cooperation with the adjacent A³, A⁴, and adjacent carbon atom. R⁶⁰ and R⁶¹ independently denote a hydrogen atom or a monovalent non-metallic atomic group. R⁶⁰ and R⁶¹ may be bonded to each other to form an aliphatic or aromatic ring.

In Formula (x), R⁶⁶ denotes an aromatic ring or a hetero ring, which may have a substituent. A⁵ denotes an oxygen atom, a sulfur atom, or —NR⁶⁷—. R⁶⁴, R⁶⁵, and R⁶⁷ independently denote a hydrogen atom or a monovalent non-metallic atomic group. R⁶⁷ and R⁶⁴, and R⁶⁵ and R⁶⁷ may be bonded to each other to form an aliphatic or aromatic ring.

In Formula (XI), A₂ denotes a sulfur atom or NR⁵⁹, L⁴ denotes a non-metallic atomic group forming a basic nucleus of a dye in cooperation with the adjacent A₂ and carbon atom, R⁵³, R⁵⁴, R⁵⁵, R⁵⁶, R⁵⁷, and R⁵⁸ independently denote a monovalent non-metallic atomic group, and R⁵⁹ denotes an alkyl group or an aryl group.

In Formula (XII), A³ and A⁴ independently denote —S—, —NR⁶²—, or —NR⁶³—, R⁶² and R⁶³ independently denote a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aryl group, L⁵ and L⁶ independently denote a non-metallic atomic group forming a basic nucleus of a dye in cooperation with the adjacent A³ and A⁴ and adjacent carbon atom, and R⁶⁰ and R⁶¹ independently denote a hydrogen atom or a monovalent non-metallic atomic group, or are bonded to each other to form an aliphatic or aromatic ring.

In Formula (XIII), R⁶⁶ denotes an aromatic ring or a hetero ring, which may have a substituent, and A⁵ denotes an oxygen atom, a sulfur atom, or —NR⁶⁷—. R⁶⁴, R⁶⁵, and R⁶⁷ independently denote a hydrogen atom or a monovalent non-metallic atomic group, and R⁶⁷ and R⁶⁴, and R⁶⁵ and R⁶⁷ may be bonded to each other to form an aliphatic or aromatic ring.

In Formula (XIV), R₆₈ and R₆₉ independently denote a hydrogen atom or a monovalent non-metallic atomic group, R₇₀ and R₇₁ independently denote a hydrogen atom or a monovalent non-metallic atomic group, and n denotes an integer of 0 to 4. When n is 2 or greater, R₇₀ and R₇₁ may be bonded to each other to form an aliphatic or aromatic ring.

Preferred examples of the compounds represented by Formulae (vi) to (XIV) include (C-1) to (C-26) below, but are not limited thereto. The structural formula of the compounds below is illustrated by a simplified structural formula, and solid lines, etc. which are not specifically designated by an element or a substituent denote a hydrocarbon group.

From the viewpoint of the coloring properties of the ink, the content of the sensitizing dye (d) in the ink composition of the present invention is preferably 0.01 to 20 wt % relative to the entire solids content of the ink composition, more preferably 0.1 to 15 wt %, and yet more preferably 0.5 to 10 wt %.

The sensitizing dye (d) may be used singly or in a combination of two or more types.

Furthermore, from the viewpoint of improvement of decomposition efficiency of the specific onium salt and transparency to irradiating light, the ratio of the specific onium salt (a) to the sensitizing dye (d) contained in the ink composition is preferably a/d=100 to 0.05 as a ratio by weight, more preferably a/d=50 to 0.1, and yet more preferably a/d=10 to 0.5.

(e) Cosensitizer

The ink composition of the present invention preferably comprises a cosensitizer. In the present invention, the cosensitizer has the function of further improving the sensitivity of the sensitizing dye to actinic radiation or the function of suppressing inhibition by oxygen of polymerization of a polymerizable compound, etc.

Examples of such a cosensitizer include amines such as compounds described in M. R. Sander et al., ‘Journal of Polymer Society’, Vol. 10, p. 3173 (1972), JP-B-44-20189, JP-A-51 -82102, JP-A-52-134692, JP-A-59-138205, JP-A-60-84305, JP-A-62-18537, JP-A-64-33104, and Research Disclosure No. 33825, and specific examples thereof include triethanolamine, ethyl p-dimethylaminobenzoate, p-formyldimethylaniline, and p-methylthiodimethylaniline.

Other examples of the cosensitizer include thiols and sulfides such as thiol compounds described in JP-A-53-702, JP-B-55-500806, and JP-A-5-142772, and disulfide compounds of JP-A-56-75643, and specific examples thereof include 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptobenzimidazole, 2-mercapto-4(3H)-quinazoline, and β-mercaptonaphthalene.

Yet other examples of the cosensitizer include amino acid compounds (e.g. N-phenylglycine, etc.), organometallic compounds described in JP-B-48-42965 (e.g. tributyltin acetate, etc.), hydrogen-donating compounds described in JP-B-55-34414, sulfur compounds described in JP-A-6-308727 (e.g. trithiane, etc.), phosphorus compounds described in JP-A-6-250387 (diethylphosphite, etc.), and Si—H or Ge—H compounds described in JP-A-8-54735.

(f) Other Polymerizable Compound

The ink composition of the present invention may employ the cationically polymerizable compound (b) and another polymerizable compound in combination. Examples of polymerizable compounds that can be used in combination in the present invention include radically polymerizable compounds. As the radically polymerizable compounds, photocurable materials employing a photopolymerizable composition described in JP-A-7-159983, JP-B-7-31399, JP-A-8-224982, JP-A-10-863, JP-A-9-80675, etc. are known (JP-B denotes a Japanese examined patent application publication).

The radically polymerizable compounds that can be used in the present invention are compounds having at least one radically polymerizable carbon-carbon double bond (ethylenically unsaturated bond), may be any compound as long as it has one radically polymerizable ethylenically unsaturated bond in the molecule, and include chemical configurations such as monomer, oligomer, and polymer. The radically polymerizable compounds may be used singly or may be used in a combination of two or more types at any ratio in order to improve desired properties.

Examples of the polymerizable compound having a radically polymerizable ethylenically unsaturated bond include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, crotonic acid, isocrotonic acid, and maleic acid, and salts thereof, anhydrides having an ethylenically unsaturated group, acrylonitrile, styrene, and various types of radically polymerizable compounds such as unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes.

Specific examples thereof include acrylic acid derivatives such as 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, butoxyethyl acrylate, carbitol acrylate, cyclohexyl acrylate, tetrahydrofurfuryl acrylate, benzyl acrylate, diacetone acrylamide, and epoxyacrylate; methacrylic derivatives such as methyl methacrylate, n-butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, allyl methacrylate, glycidyl methacrylate, benzyl methacrylate, and dimethylaminomethyl methacrylate; and allyl compound derivatives such as allyl glycidyl ether. More specifically, commercial products, radically polymerizable or crosslinking monomers, oligomers, and polymers known in the art such as those described in ‘Kakyozai Handobukku’ (Crosslinking Agent Handbook), Ed. S. Yamashita (Taiseisha, 1981); ‘UV.EB Koka Handobukku’ (UV.EB Curing Handbook (Starting Materials) Ed. K. Kato (Kobunshi Kankoukai, 1985); ‘UV.EB Koka Gijutsu no Oyo to Shijyo’ (Application and Market of UV.EB Curing Technology', p. 79, Ed. Rad Tech (CMC, 1989); and E. Takiyama ‘Poriesuteru Jushi Handobukku’ (Polyester Resin Handbook), (The Nikkan Kogyo Shimbun Ltd., 1988) can be used.

It is preferable to use the radically polymerizable compound and the cationically polymerizable compound in combination since, due to high sensitivity characteristic of radical polymerization and low volume shrinkage characteristic of cationic polymerization, a printed material or a lithographic printing plate having both high sensitivity and adhesion is obtained.

(g) Another Polymerization Initiator

The ink composition of the present invention may employ (a) the specific onium salt and (g) another polymerization initiator in combination. As the polymerization initiator employed in combination, known radical polymerization initiators can be cited. Said other polymerization initiators may be used singly or in a combination of two or more types.

The radical polymerization initiator that can be used in the ink composition of the present invention is a compound that forms a polymerization initiating species by absorbing external energy. The external energy used for initiating polymerization can be broadly divided into heat and actinic radiation, and a thermal polymerization initiator and a photopolymerization initiator are used respectively. Examples of the actinic radiation include y rays, P rays, an electron beam, UV rays, visible light, and IR rays. The polymerization initiator that can be used in the present invention is preferably a radiation-sensitive radical polymerization initiator that is sensitive to actinic radiation, that is, a so-called radical photopolymerization initiator.

Examples of the radical polymerization initiator that can be used in combination in the present invention include (a) aromatic ketones, (b) organic peroxides, (c) thio compounds, (d) hexaarylbiimidazole compounds, (e) ketoxime ester compounds, (f) borate compounds, (g) azinium compounds, (h) metallocene compounds, (i) active ester compounds, (j) compounds having a carbon-halogen bond, and (k) alkylamine compounds. These radical polymerization initiators may be used singly or in a combination of the above-mentioned compounds (a) to (k).

(h) Other Components

The ink composition of the present invention may comprise other components as necessary. Examples of the other components include basic compounds, polymerization inhibitors, solvents, and cationically polymerizable monomers.

It is preferable to add the basic compound from the viewpoint of improving the storage stability of the ink composition. As the basic compound that can be used in the present invention, a known basic compound may be used and, for example, a basic inorganic compound such as an inorganic salt or a basic organic compound such as an amine is preferably used.

It is preferable to add the polymerization inhibitor from the viewpoint of enhancing the storage stability. When the ink composition of the present invention is used as an inkjet recording ink composition, it is preferably heated in the range of 40° C. to 80° C. to thus make it less viscous and then discharged, and in order to prevent clogging of a head due to thermal polymerization it is preferable to add a polymerization inhibitor. The polymerization inhibitor is preferably added at 200 to 20,000 ppm relative to the total amount of the ink composition of the present invention. Examples of the polymerization inhibitor include hydroquinone, benzoquinone, p-methoxyphenol, TEMPO, TEMPOL, and Al cupferron.

While taking into consideration the ink composition and the inkjet recording ink composition of the present invention being radiation curing type ink compositions, it is preferable for them not to contain any solvent so that the ink compositions can react quickly and be cured immediately after landing. However, as long as the curing speed, etc. of the ink composition is not affected, a specified solvent may be added. In the present invention, as a solvent, an organic solvent or water may be used. In particular, the organic solvent may be added in order to improve the adhesion to a recording medium (a support such as paper). Adding an organic solvent having a high boiling point is effective since the problem with VOC can be avoided. The amount of organic solvent is preferably 0.1 to 5 wt % relative to the total amount of the ink composition of the present invention, and more preferably 0.1 to 3 wt %.

As means for preventing the sensitivity from being degraded by a light blocking effect of the coloring agent, which may be added to the ink composition, a combination of a cationically polymerizable compound and a cationic polymerization initiator, a combination of a radically polymerizable compound and a radical polymerization initiator, or a radical/cationic hybrid curing ink combining a polymerizable compound and a polymerization initiator may be employed.

In addition to the above, the ink composition of the present invention may contain a known compound as necessary. Examples thereof include a surfactant (cationic, anionic, nonionic, fluorine-based, silicon-based), a leveling additive, a matting agent and, for adjusting film physical properties, a polyester resin, polyurethane resin, vinyl resin, acrylic resin, rubber resin, or wax, which may be appropriately selected and added. Furthermore, in order to improve the adhesion to a recording medium such as a polyolefin or PET, a tackifier that does not inhibit polymerization is preferably added. Specific examples of the tackifier include high molecular weight tacky polymers described on pp. 5 and 6 of JP-A-2001-49200 (e.g. a copolymer formed from an ester of (meth)acrylic acid and an alcohol having an alkyl group with 1 to 20 carbons, an ester of (meth)acrylic acid and an alicyclic alcohol having 3 to 14 carbons, or an ester of (meth)acrylic acid and an aromatic alcohol having 6 to 14 carbons), and a low molecular weight tackifying resin having a polymerizable unsaturated bond.

(2) Properties of Ink Composition

The ink composition of the present invention comprises as described above (a) the specific onium salt, and (b) a cationically polymerizable compound, and as necessary (c) a colorant. With regard to these components, relative to the total solids content of the ink composition, (a) the specific onium salt is preferably 1 to 20 wt %, and more preferably 2 to 15 wt %, (b) a cationically polymerizable compound is preferably 10 to 95 wt %, more preferably 30 to 90 wt %, and further more preferably 50 to 85 wt %, and in a case which a colorant is used, (c) the colorant is preferably 1 to 10 wt %, and more preferably 2 to 8 wt %, and the total wt % including other optional components becomes 100 wt %.

When the ink composition thus obtained is used for inkjet recording, while taking into consideration dischargability, the viscosity of the ink composition at the discharge temperature (e.g. 25° C. to 80° C., preferably 25° C. to 40° C., and more preferably 25° C to 30° C.) is preferably 5 to 30 mPa s, and more preferably 7 to 15 mPa.s. For example, the ink composition of the present invention has a viscosity at room temperature (25° C. to 30° C.) of preferably 8 to 300 mPa.s, and more preferably 10 to 100 mPa.s. With regard to the ink composition of the present invention, it is preferable that its component ratio is appropriately adjusted so that the viscosity is in the above-mentioned range. When the viscosity at room temperature is set to be high, even when a porous recording medium is used, penetration of the ink into the recording medium can be prevented, uncured monomer can be reduced, and the odor can be reduced. Furthermore, ink spreading when ink droplets have landed can be suppressed, and as a result there is the advantage that the image quality is improved. When the viscosity at room temperature is set to be low, since it is unnecessary to heat the ink when discharging or it is possible to set the heating temperature at a relatively low temperature, there are the advantages that the load on inkjet equipment becomes small and the choice of inkjet heads that can be used is widened.

The surface tension of the ink composition of the present invention is preferably 20 to 30 mN/m, and yet more preferably 23 to 28 mN/m. When recording is carried out on various types of recording medium such as polyolefin, PET, coated paper, and uncoated paper, from the viewpoint of spread and penetration, it is preferably at least 20 mN/m, and from the viewpoint of wettability it is preferably not more than 30 mN/m.

(3) Inkjet Recording Method and Equipment

The ink composition of the present invention is preferably used for inkjet recording.

An inkjet recording method that can be suitably employed in the present invention is explained below.

(3-1) Inkjet Recording Method

The present invention provides a method for forming an image by discharging the above-mentioned ink composition onto a recording medium (support, recording material, etc.) and curing the ink composition by irradiating the ink composition so discharged onto the recording medium with actinic radiation. That is, the present invention relates to an inkjet recording method comprising: (a′) a step of discharging an ink composition onto a recording medium; and (b′) a step of curing the ink composition by irradiating the ink composition so discharged with actinic radiation,

wherein the ink composition is the ink composition of the present invention.

The cured ink composition forms an image on the recording medium.

The peak wavelength of the actinic radiation is preferably 200 to 600 nm, more preferably 300 to 450 nm, and yet more preferably 350 to 420 nm. The output of the actinic radiation is preferably no greater than 2,000 mJ/cm², and is more preferably 10 to 2,000 mJ/cm², yet more preferably 20 to 1,000 mJ/cm², and particularly preferably 50 to 800 mJ/cm².

The inkjet recording method of the present invention is explained by taking as an example a process for producing a lithographic printing plate, the process comprising discharging an ink composition onto the lithographic printing plate so as to form an image.

A process for producing a lithographic printing plate of the present invention comprises:

-   (a″) a step of discharging the ink composition of the present     invention onto a hydrophilic support, and -   (b″) a step of irradiating the discharged ink composition with     radiation so as to cure the ink composition, thus forming a     hydrophobic image on the hydrophilic support by curing the ink     composition.     (3-1-1) Hydrophilic Support Used for Lithographic Printing Plate

The lithographic printing plate comprises a support and an image formed on the support.

Conventionally, as the lithographic printing plate, a so-called PS plate in which an oleophilic photosensitive resin layer is provided on a hydrophilic support has been widely used. In a process for producing this PS plate, normally, after a mask exposure (surface exposure) is carried out via a lith film, non-exposed areas are dissolved and removed to give a desired printing plate. However, in recent years, a technique of digitizing image information using a computer by electronically processing, storing, and outputting the information has become widespread, and a new image output system that can be used for the above technique has been desired. In particular, a computer to plate (CTP) technique in which a printing plate is directly produced by scanning according to digitized image information with highly coherent light such as laser light without using a lith film has been developed.

As a system for obtaining a lithographic printing plate that makes possible the above scanning exposure, a process for directly producing a lithographic printing plate using an ink composition or an inkjet recording ink composition can be cited. This process involves obtaining a printing plate having a desired image (preferably a hydrophobic image) by discharging an ink onto a support, and preferably a hydrophilic support, using an inkjet system, etc., and exposing this to actinic radiation so as to expose an area with the ink composition or the inkjet recording ink to light. The ink composition or the inkjet recording ink suitable for such a system is the ink composition or the inkjet recording ink of the present invention.

The support (recording medium) onto which the ink composition or the inkjet recording ink composition of the present invention is discharged is not particularly limited, and a dimensionally stable sheet-form support may be used. The support is preferably a hydrophilic support. The support used in the lithographic printing plate of the present invention is not particularly limited, and a dimensionally stable sheet-form support may be used. It is preferable that a material forming the support has a hydrophilic surface. Examples of materials forming the support include paper, paper laminated with a plastic (e.g. polyethylene, polypropylene, polystyrene, etc.), a metal sheet (e.g. aluminum, zinc, copper, etc.), a plastic film (e.g. cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate butyrate, cellulose nitrate, polyethylene terephthalate, polyethylene, polystyrene, polypropylene, polycarbonate, polyvinyl acetal, etc.), and paper or plastic film on which the above-mentioned metal is laminated or vapor-deposited. Preferred examples of the support include a polyester film and aluminum sheet. Among these, aluminum sheet is particularly preferable since the dimensional stability is good and it is relatively inexpensive.

The aluminum sheet is a pure aluminum sheet, an alloy sheet containing aluminum as a main component and a small amount of a different element, or a thin film of aluminum or an aluminum alloy laminated with a plastic. Examples of the different element contained in the aluminum alloy include silicon, iron, manganese, copper, magnesium, chromium, zinc, bismuth, nickel, and titanium. The content of the different element in the alloy is preferably equal to or less than 10 wt %. In the present invention, a pure aluminum sheet is preferable, but since it is difficult to produce completely pure aluminum because of the refining technique, a trace amount of a different element may be contained. The composition of the aluminum sheet is not specified, and a known generally used material may be utilized as appropriate.

The support preferably has a thickness of 0.1 to 0.6 mm, and more preferably 0.15 to 0.4 mm.

Prior to the aluminum sheet being used, it is preferably subjected to a surface treatment such as a surface roughening treatment or an anodizing treatment. Surface treatment makes it easy to improve the hydrophilicity and ensure that there is good adhesion between an image recording layer and the support. Prior to the aluminum sheet being subjected to the surface roughening treatment, it may be subjected as desired to a degreasing treatment using a surfactant, an organic solvent, an aqueous alkaline solution, etc. in order to remove rolling oil on the surface.

The surface roughening treatment for the aluminum sheet surface may be carried out by various types of methods, and examples thereof include a mechanical surface roughening treatment, an electrochemical surface roughening treatment (a surface roughening treatment involving dissolving the surface electrochemically), and a chemical surface roughening treatment (a surface roughening treatment involving selectively dissolving the surface chemically).

As a method for the mechanical surface roughening treatment, a known method such as a ball grinding method, a brush grinding method, a blast grinding method, or a buff grinding method may be used. It is also possible to use a transfer method in which an irregular shape is transferred using a roller provided with irregularities in an aluminum rolling stage.

As a method for the electrochemical surface roughening treatment, for example, a method in which alternating current or direct current is applied in an electrolyte solution containing an acid such as hydrochloric acid or nitric acid can be cited. It is also possible to employ a method as described in JP-A-54-63902 in which a mixed acid is used.

The aluminum sheet subjected to a surface roughening treatment is subjected as necessary to an alkali etching treatment using an aqueous solution of potassium hydroxide, sodium hydroxide, etc.; furthermore, after neutralization, it may be subjected to an anodizing treatment as desired in order to improve the abrasion resistance.

As an electrolyte that may be used for the anodizing treatment of the aluminum sheet, various types of electrolytes that form a porous oxide film may be used. In general, sulfuric acid, hydrochloric acid, oxalic acid, chromic acid, or a mixed acid thereof may be used. The concentration of the electrolyte may be determined as appropriate according to the type of electrolyte.

Conditions for the anodizing treatment depend on the type of electrolyte used and cannot be specified, but in general the electrolyte solution concentration is 1 to 80 wt %, the solution temperature is 5° C. to 70° C., the current density is 5 to 60 A/dm², the voltage is 1 to 100V, and the electrolysis time is 10 sec. to 5 min. The amount of anodized film formed is preferably 1.0 to 5.0 g/m², and more preferably 1.5 to 4.0 g/m². It is preferable for it to be in this range since good plate life and good scratch resistance of a non-image area of a lithographic printing plate can be obtained.

As the support that can be used in the present invention, a substrate that has been subjected to the above-mentioned surface treatment and has an anodized film may be used as it is, but in order to further improve the adhesion to an upper layer, and the hydrophilicity, the contamination resistance, the thermal insulation, etc., the substrate may appropriately be subjected as necessary to a treatment for enlarging micropores of the anodized film, a sealing treatment, or a surface hydrophilization treatment involving immersion in an aqueous solution containing a hydrophilic compound, which are described in JP-A-2001-253181 or JP-A-2001-322365. These enlarging and sealing treatments are not limited to those described therein, and any conventionally known methods may be employed.

Sealing Treatment

The sealing treatment may be vapor sealing, a treatment with an aqueous solution containing an inorganic fluorine compound such as a single treatment with fluorozirconic acid or a treatment with sodium fluoride, vapor sealing with added lithium chloride, or a sealing treatment with hot water.

Among these, the sealing treatment with an aqueous solution containing an inorganic fluorine compound, the sealing treatment with vapor, and the sealing treatment with hot water are preferable. Each thereof is explained below.

<Sealing Treatment With Aqueous Solution Containing Inorganic Fluorine Compound>

In the sealing treatment with an aqueous solution containing an inorganic fluorine compound, a metal fluoride can suitably be used as the inorganic fluorine compound.

Specific examples thereof include sodium fluoride, potassium fluoride, calcium fluoride, magnesium fluoride, sodium fluorozirconate, potassium fluorozirconate, sodium fluorotitanate, potassium fluorotitanate, ammonium fluorozirconate, ammonium fluorotitanate, potassium fluorotitanate, fluorozirconic acid, fluorotitanic acid, hexafluorosilicic acid, nickel fluoride, iron fluoride, fluorophosphoric acid, and ammonium fluorophosphate. Among them, sodium fluorozirconate, sodium fluorotitanate, fluorozirconic acid, and fluorotitanic acid are preferable.

The concentration of the inorganic fluorine compound in the aqueous solution is preferably at least 0.01 wt % from the viewpoint of sealing of micropores on an anodized coating being carried out sufficiently, and more preferably at least 0.05 wt %, and it is preferably no greater than 1 wt % from the viewpoint of contamination resistance, and more preferably no greater than 0.5 wt %.

The aqueous solution containing an inorganic fluorine compound preferably further contains a phosphate compound. It is preferable for a phosphate compound to be contained since the hydrophilicity of the surface of the anodized coating improves and the machine developability and the contamination resistance can be improved.

Preferred examples of the phosphate compound include phosphates of a metal such as an alkali metal or an alkaline earth metal.

Specific examples thereof include zinc phosphate, aluminum phosphate, ammonium phosphate, diammonium hydrogenphosphate, ammonium dihydrogenphosphate, potassium dihydrogenphosphate, dipotassium hydrogenphosphate, calcium phosphate, ammonium sodium hydrogenphosphate, magnesium hydrogenphosphate, magnesium phosphate, ferrous phosphate, ferric phosphate, sodium dihydrogenphosphate, sodium phosphate, disodium hydrogenphosphate, lead phosphate, diammonium phosphate, calcium dihydrogenphosphate, lithium phosphate, phosphotungstic acid, ammonium phosphotungstate, sodium phosphotungstate, ammonium phosphomolybdate, sodium phosphomolybdate, sodium phosphite, sodium tripolyphosphate, and sodium pyrophosphate. Among these, sodium dihydrogenphosphate, disodium hydrogenphosphate, potassium dihydrogenphosphate, and dipotassium hydrogenphosphate are preferable.

The combination of the inorganic fluorine compound and the phosphate compound is not particularly limited, but the aqueous solution preferably comprises at least sodium fluorozirconate as the inorganic fluorine compound and at least sodium dihydrogen phosphate as the phosphate compound.

The concentration of the phosphate compound in the aqueous solution is preferably at least 0.01 wt % from the viewpoint of improving machine developability and contamination resistance, and more preferably at least 0.1 wt %, and it is preferably no greater than 20 wt % from the viewpoint of solubility, and more preferably no greater than 5 wt %.

The proportion of each compound in the aqueous solution is not particularly limited, but the ratio by weight of the inorganic fluorine compound and the phosphate compound is preferably 1/200 to 10/1, and more preferably 1/30 to 2/1.

Furthermore, the temperature of the aqueous solution is preferably at least 20° C., and more preferably at least 40° C., and it is preferably no higher than 1 00C, and more preferably no higher than 80° C.

Moreover, the pH of the aqueous solution is preferably at least 1, and more preferably at least 2, and it is preferably no greater than 11, and more preferably no greater than 5.

A method for the sealing treatment with the aqueous solution containing an inorganic fluorine compound is not particularly limited and, for example, an immersion method and a spray method may be used. They may be employed once or a plurality of times, or in a combination of two or more types.

Among these, the immersion method is preferable. When the treatment is carried out by the immersion method, the treatment time is preferably at least 1 sec., and more preferably at least 3 sec., and it is preferably no greater than 100 sec., and more preferably no greater than 20 sec.

<Sealing Treatment With Steam>

With regard to the sealing treatment with steam, for example, a method in which an anodized coating is contacted with steam at high pressure or normal pressure continuously or discontinuously can be cited.

The temperature of the steam is preferably at least 80° C., and more preferably at least 95° C., and it is preferably no greater than 1 05° C.

The pressure of the steam is preferably in the range of (atmospheric pressure−50 mmAq) to (atmospheric pressure+300 mmAq) (1.008×10⁵ to 1.043×10⁵ Pa).

Furthermore, the time for which the coating is contacted with steam is preferably at least 1 sec., and more preferably at least 3 sec., and it is preferably no greater than 100 sec., and more preferably no greater than 20 sec.

<Sealing Treatment With Hot Water>

With regard to the sealing treatment with hot water, for example, a method in which an aluminum plate having an anodized coating formed thereon is immersed in hot water can be cited.

The hot water may contain an inorganic salt (e.g. a phosphate) or an organic salt.

The temperature of the hot water is preferably at least 80° C., and more preferably at least 95° C., and it is preferably no greater than 100° C.

Furthermore, the time for which immersion in hot water is carried out is preferably at least 1 sec., and more preferably at least 3 sec., and it is preferably no greater than 100 sec., and more preferably no greater than 20 sec.

With regard to a hydrophilization treatment that is used in the present invention, there is an alkali metal silicate method, as disclosed in U.S. Pat. Nos. 2,714,066, 3,181,461, 3,280,734, and 3,902,734. In this method, a support is immersed in an aqueous solution of sodium silicate, etc., or subjected to electrolysis. In addition, there is a method in which a support is treated with potassium fluorozirconate, as described in JP-B-36-22063, and a method in which a support is treated with polyvinylphosphonic acid, as described in U.S. Pat. Nos. 3,276,868, 4,153,461, and 4,689,272.

In the present invention, it is preferable for the support to have a center line average roughness of 0.10 to 1.2 μm. It is preferable for it to be in this range since good adhesion to an image recording layer, good plate life, and good contamination resistance can be obtained.

(3-1-2) Step of Discharging Ink Composition Onto Hydrophilic Support

When the ink composition or the inkjet recording ink composition of the present invention is discharged onto the surface of the above-mentioned hydrophilic support, the ink composition or the inkjet recording ink composition is preferably discharged after being heated to preferably 40° C. to 80° C., and more preferably 25° C. to 30° C., so as to reduce the viscosity of the ink composition to preferably 7 to 30 mPa.s, and more preferably 7 to 20 mPa s. In particular, it is preferable to use the ink composition having an ink viscosity at 25° C of 35 to 500 mPa.s since a large effect can be obtained. By employing this method, high discharge stability can be realized. The radiation curing type ink composition such as the ink composition of the present invention generally has a viscosity that is higher than that of a normal ink composition or a water-based ink used for an inkjet recording ink, and variation in viscosity due to a change in temperature at the time of discharge is large. Viscosity variation in the ink has a large effect on changes in liquid droplet size and changes in liquid droplet discharge speed and, consequently, causes the image quality to be degraded. It is therefore necessary to maintain the ink discharge temperature as constant as possible. In the present invention, the control range for the temperature is desirably ±5° C. of a set temperature, preferably ±2° C. of the set temperature, and more preferably ±1° C. of the set temperature.

(3-1-3) Step of Curing Ink Composition by Irradiating Discharged Ink Composition With Actinic Radiation so as to Form Hydrophobic Image on Hydrophilic Support by Curing Ink Composition

The ink composition discharged onto the surface of the hydrophilic support is cured by irradiating with actinic radiation. This results from a sensitizing dye in a polymerization initiation system contained in the above-mentioned ink composition of the present invention absorbing actinic radiation, attaining an excited state, and coming into contact with a polymerization initiator in the polymerization initiation system to thus decompose the polymerization initiator, and a polymerizable compound undergoing radical polymerization and being cured.

The actinic radiation used in this process may include α rays, γ rays, an electron beam, X rays, UV rays, visible light, and IR rays. Although it depends on the absorption characteristics of the sensitizing dye, the peak wavelength of the actinic radiation is, for example, 200 to 600 nm, preferably 300 to 450 nm, and more preferably 350 to 450 nm. Furthermore, in the present invention, the polymerization initiation system has sufficient sensitivity for low output actinic radiation. The output of the actinic radiation as irradiation energy is therefore, for example, 2,000 mJ/cm² or less, and is preferably 10 to 2,000 mJ/cm², more preferably 20 to 1,000 mJ/cm², and yet more preferably 50 to 800 mJ/cm². Moreover, the actinic radiation is applied so that the illumination intensity on the exposed surface is, for example, 10 to 2,000 mW/cm², and preferably 20 to 1,000 mW/cm².

The ink composition of the present invention is desirably exposed to such actinic radiation for, for example, 0.01 to 120 sec., and preferably 0.1 to 90 sec.

Irradiation conditions and a basic method for irradiation with actinic radiation are disclosed in JP-A-60-132767. Specifically, a light source is provided on either side of a head unit that includes an ink discharge device, and the head unit and the light source are made to scan by a so-called shuttle system. Irradiation with actinic radiation is carried out after a certain time (e.g. 0.01 to 0.5 sec., preferably 0.01 to 0.3 sec., and more preferably 0.01 to 0.15 sec.) has elapsed from when the ink has landed. By controlling the time from ink landing to irradiation so as to be a minimum in this way, it becomes possible to prevent the ink that has landed on a recording medium from spreading before being cured. Furthermore, since the ink can be exposed before it reaches a deep area of a porous recording medium that the light source cannot reach, it is possible to prevent monomer from remaining unreacted, and as a result the odor can be reduced.

Furthermore, curing may be completed using another light source that is not driven. WO99/54415 discloses, as an irradiation method, a method employing an optical fiber and a method in which a collimated light source is incident on a mirror surface provided on a head unit side face, and a recorded area is irradiated with UV light.

By employing such a recording method, it is possible to maintain a uniform dot diameter for landed ink even for various types of recording media having different surface wettability, thereby improving the image quality. In order to obtain a color image, it is preferable to superimpose colors in order from those with a low lightness. By superimposing inks in order from one with low lightness, it is easy for radiation to reach a lower ink, the curing sensitivity is good, the amount of residual monomer decreases, odor is reduced, and an improvement in adhesion can be expected. Furthermore, although it is possible to discharge all colors and then expose them at the same time, it is preferable to expose one color at a time from the viewpoint of promoting curing.

In this way, the above-mentioned ink composition of the present invention is cured by irradiation with actinic radiation to thus form a hydrophobic image on the surface of the hydrophilic support.

(3-2) Inkjet Recording Device

The inkjet recording device used in the present invention is not particularly restricted, and a commercial inkjet recording device may be used. That is, in the present invention, recording on a recording medium may be carried out using a commercial inkjet recording device.

The inkjet recording device that can be used in the present invention is equipped with, for example, an ink supply system, a temperature sensor, and an actinic radiation source.

The ink supply comprises, for example, a main tank containing the ink composition of the present invention, a supply pipe, an ink supply tank immediately before an inkjet head, a filter, and a piezo system inkjet head. The piezo system inkjet head may be driven so as to discharge a multisize dot of 1 to 100 pL, and preferably 8 to 30 pL, at a resolution of 320×320 to 4,000×4,000dpi, preferably 400×400 to 1,600×1,600dpi, and more preferably 720×720 dpi. Here, dpi referred to in the present invention means the number of dots per 2.54 cm.

As described above, since it is desirable for the radiation curing type ink to be discharged at a constant temperature, a section from the ink supply tank to the inkjet head is thermally insulated and heated. A method of controlling temperature is not particularly limited, but it is preferable to provide, for example, temperature sensors at a plurality of pipe section positions, and control heating according to the ink flow rate and the temperature of the surroundings. The temperature sensors may be provided on the ink supply tank and in the vicinity of the inkjet head nozzle. Furthermore, the head unit that is to be heated is preferably thermally shielded or insulated so that the device main body is not influenced by the temperature of the outside air. In order to reduce the printer start-up time required for heating, or in order to reduce the thermal energy loss, it is preferable to thermally insulate the head unit from other sections and also to reduce the heat capacity of the entire heated unit.

As an actinic radiation source, a mercury lamp, a gas/solid laser, etc. are mainly used, and for UV photocuring inkjet a mercury lamp and a metal halide lamp are widely known. However, from the viewpoint of protection of the environment, there has recently been a strong desire for mercury not to be used, and replacement by a GaN semiconductor UV light emitting device is very useful from industrial and environmental viewpoints. Furthermore, LEDs (UV-LED) and LDs (UV-LD) have small dimensions, long life, high efficiency, and low cost, and their use as a photocuring inkjet light source can be expected.

Furthermore, light-emitting diodes (LED) and laser diodes (LD) may be used as the source of actinic radiation. In particular, when a UV ray source is needed, a UV-LED or a UV-LD may be used. For example, Nichia Corporation has marketed a violet LED having a wavelength of the main emission spectrum of between 365 nm and 420 nm. Furthermore, when a shorter wavelength is needed, U.S. Pat. No. 6,084,250 discloses an LED that can emit actinic radiation whose wavelength is centered between 300 nm and 370 nm. Furthermore, another violet LED is available, and irradiation can be carried out with radiation of a different UV bandwidth. The actinic radiation source particularly preferable in the present invention is a UV-LED, and a UV-LED having a peak wavelength at 350 to 420 nm is particularly preferable.

The maximum illumination intensity of the LED on a recording medium is preferably 10 to 2,000 mW/cm², more preferably 20 to 1,000 mW/cm², and particularly preferably 50 to 800 mJ/cm².

In accordance with the present invention, it is possible to provide an ink composition that cures with high sensitivity when exposed to radiation, can form a high quality image, has excellent adhesion to a recording medium, and has good storage stability, and an inkjet recording meihod employing the ink composition.

Furthermore, a printed material obtained using the ink composition having excellent storage stability, and being capable of curing with high sensitivity when exposed to radiation has high image quality and excellent strength for an image area. Similarly, in accordance with use of the ink composition of the present invention, there is exhibited the effect that a lithographic printing plate having a long plate life and high image quality can be produced based on digital data.

EXAMPLES

The present invention is explained in further detail by reference to Examples and Comparative Examples. However, the present invention should not be construed as being limited to these Examples.

Preparation of Pigment Dispersions

Yellow, magenta, cyan, and black pigment dispersions 1 were prepared in accordance with the method described below. Dispersion conditions were appropriately adjusted using a known dispersing machine so that the average particle size of pigment particles was in the range of 0.2 to 0.3 pm, and they were subsequently filtered using a filter while heating. Yellow pigment dispersion 1 CI Pigment Yellow 13 20 parts by weight Polymeric dispersant 20 parts by weight (Solsperse series manufactured by Zeneca) Diethylene glycol divinyl ether 60 parts by weight (manufactured by BASF)

Magenta pigment dispersion 1 CI Pigment Red 57:1 20 parts by weight Polymeric dispersant 20 parts by weight (Solsperse series manufactured by Zeneca) Diethylene glycol divinyl ether 60 parts by weight (manufactured by BASF)

Cyan pigment dispersion 1 CI Pigment Blue 15:3 20 parts by weight Polymeric dispersant 20 parts by weight (Solsperse series manufactured by Zeneca) Diethylene glycol divinyl ether 60 parts by weight (manufactured by BASF)

Black pigment dispersion 1 CI Pigment black 7 20 parts by weight Polymeric dispersant 20 parts by weight (Solsperse series manufactured by Zeneca) Diethylene glycol divinyl ether 60 parts by weight (manufactured by BASF) Preparation of Inks

Example 1

Yellow ink 1 Yellow pigment dispersion 1  5 parts by weight Compound A below (counteranion volume:  6 parts by weight 2,091 Å³) Sensitizing dye: 9,10-dibutoxyanthracene  3 parts by weight Polymerizable compounds Monomer: 3,4-epoxycyclohexylmethyl-3′, 40 parts by weight 4′-epoxycyclohexanecarboxylate (Celloxide 2021A: manufactured by Daicel-UCB Co., Ltd.) Monomer: 3,7-bis(3-oxetanyl)-5-oxanonane (OXT- 45 parts by weight 221: manufactured by Toagosei Co., Ltd.) Basic compound: a-1 (Amine-1 below)  1 parts by weight

Compound A

Amine-1 (Compound described in JP-A-2003-305839)

Magenta ink 1 Magenta pigment dispersion 1 5 parts by weight Compound A 6 parts by weight Polymerizable compounds Monomer: 3,4-epoxycyclohexylmethyl-3′,4′- 40 parts by weight epoxycyclohexanecarboxylate (Celloxide 2021A: manufactured by Daicel-UCB Co., Ltd.) Monomer: 3,7-bis(3-oxetanyl)-5-oxanonane 48 parts by weight (OXT-221: manufactured by Toagosei Co., Ltd.) Basic compound: a-1 1 part by weight

Cyan ink 1 Cyan pigment dispersion 1 5 parts by weight Compound A 6 parts by weight Polymerizable compounds Monomer: 3,4-epoxycyclohexylmethyl-3′,4′- 40 parts by weight epoxycyclohexanecarboxylate (Celloxide 2021A: manufactured by Daicel-UCB Co., Ltd.) Monomer: 3,7-bis(3-oxetanyl)-5-oxanonane 48 parts by weight (OXT-221: manufactured by Toagosei Co., Ltd.) Basic compound: a-1 1 part by weight

Black ink 1 Black pigment dispersion 1 5 parts by weight Compound A 6 parts by weight Polymerizable compounds Monomer: 3,4-epoxycyclohexylmethyl-3′,4′- 40 parts by weight epoxycyclohexanecarboxylate (Celloxide 2021A: manufactured by Daicel-UCB Co., Ltd.) Monomer: 3,7-bis(3-oxetanyl)-5-oxanonane 48 parts by weight (OXT-221: manufactured by Toagosei Co., Ltd.) Basic compound: a-1 1 part by weight Inks 1 of each color prepared above were filtered using a filter having an absolute filtration accuracy of 2 μm to give Inks 1 of each color. Inkjet Image Recording

Subsequently, recording was carried out on a recording medium using a commercial inkjet recording device having a piezo system inkjet nozzle. The ink supply system comprised a main tank, a supply pipe, an ink supply tank immediately before an inkjet head, a filter, and a piezo system inkjet head, and a section from the ink supply tank to the inkjet head was thermally insulated and heated. Temperature sensors were provided on the ink supply tank and in the vicinity of the nozzle of the inkjet head, and the temperature was controlled so that the nozzle section was always at 70° C.±2° C. The piezo system inkjet head was driven so as to discharge a multisize dot of 8 to 30 pL at a resolution of 720×720 dpi. The exposure system, the main scanning speed, and the discharge frequency were adjusted so that, after landing, UV-A light was focused to give an exposure area illumination intensity of 100 mW/cm², and irradiation started 0.1 sec. after the ink landed on the recording medium. Furthermore, the exposure time was made variable, and exposure energy was applied. Here, dpi referred to in the present invention denotes the number of dots per 2.54 cm.

The inks of each color prepared above were discharged in the order black, cyan, magenta, and yellow at an environmental temperature of 25° C., and each color was irradiated with UV rays using a VZero 085 metal halide lamp manufactured by Integration Technology. As an energy level that could completely cure the inks so that tackiness disappeared when touched by hand, the total exposure energy per color was 100 mJ/cm² for all the colors. As recording media, a grained aluminum support, a transparent biaxially stretched polypropylene film whose surface had been treated so as to impart printability, a soft vinyl chloride sheet, a cast coat paper, and a commercial recycled paper were used, each color image was recorded, and an image having high resolution without dot spreading was obtained in all cases. Furthermore, for high quality paper, the ink did not penetrate to the reverse side, the ink was sufficiently cured, and there was hardly any odor due to unreacted monomer. Moreover, the ink recorded on the film had sufficient flexibility, the ink did not crack when bent, and there was no problem in an adhesion test involving peeling with Sellotape (registered trademark).

Examples 2 to 9

Preparation of Inks

Magenta inks 2 to 9 were prepared in accordance with the method described below.

Magenta inks 2 to 9

Magenta inks 2 to 9 were prepared in the same manner as for the above magenta ink 1 except that the specific onium salt and other additional components were changed to those described in the table below. TABLE 2 Volume of counteranion Magenta of specific ink No. (a) Specific onium salt onium salt (a) Added component Ex. 2 2

1,514 Ex. 3 3

2,091 Ex. 4 4

2,091 Ex. 5 5

2,091 Ex. 6 6

1,298 Ex. 7 7

3,897 Ex. 8 8

1,020 Ex. 9 9

2,091 9,10- Dibutoxyanthracene 3 parts by weight

Example 10: Magenta ink 10 Magenta pigment dispersion 1 5 parts by weight The above compound A 6 parts by weight Polymerizable compounds Cationically polymerizable monomer: 3,4- 30 parts by weight epoxycyclohexylmethyl-3′,4′- epoxycyclohexanecarboxylate (Celloxide 2021A: manufactured by Daicel-UCB Co., Ltd.) Cationically polymerizable monomer: 3,7-bis 48 parts by weight (3-oxetanyl)-5-oxanonane (OXT-221: manufactured by Toagosei Co., Ltd.) Radically polymerizable monomer: 1,6-hexanediol 10 parts by weight diacrylate Basic compound: a-1 1 part by weight

Magenta ink 10 prepared as above was filtered using a filter having an absolute filtration accuracy of 2 μm to give magenta ink 10.

Comparative Examples 1 to 4

Preparation of Ink

Magenta inks 11 to 14 were prepared in the same manner as for the above magenta ink 1 except that the cationic polymerization initiator was changed to those described in the table below. TABLE 3 Volume of counteranion of specific Magenta onium salt (a) ink No. (a) Specific onium salt (Å³) Comp. Ex. 1 11

95 Comp. Ex. 2 12

127 Comp. Ex. 3 13

537 Comp. Ex. 4 14

783

Magenta inks 11 to 14 prepared as above were filtered using a filter having an absolute filtration accuracy of 2 μm to give Magenta inks 1 1 to 14.

Example 11

Magenta ink 15 was prepared in the same manner as for magenta ink 1 except that Compound M-1 below (oxidation potential +1.37 V) was used as an oil-soluble dye instead of CI pigment red 57:1, and filtration was carried out using a filter having an absolute filtration accuracy of 2 μm.

Example 12

Magenta ink 16 was prepared in the same manner as for magenta ink 1 except that Compound M-2 below (oxidation potential +0.94 V) was used as an oil-soluble dye instead of CI pigment red 57:1, and filtration was carried out using a filter having an absolute filtration accuracy of 2 μm.

Inkjet Image Recording

Magenta images were produced in the same manner as in the method described in Example 1 using the magenta inks 2 to 16 prepared as above.

Example 13

A magenta image was formed using Magenta ink 9 by the same method as in Example 1 except that a UV light-emitting diode (UV-LED) was used instead of the VZero 085 metal halide lamp manufactured by Integration Technology.

In this embodiment, an NCCU033 manufactured by Nichia Corporation was used as the UV-LED. The LED emits UV light at a wavelength of 365 nm from 1 chip, and by applying a current of about 500 mA, light of about 100 mW is emitted from the chip. A plurality thereof were aligned at intervals of 7 mm to give a power of 0.3 W/cm² on the surface of a recording medium (hereinafter, also called a medium). The time from landing to irradiation and the exposure time can be varied by the transport speed of the medium and the distance between a head and the LED in the transport direction. In this embodiment, irradiation was carried out about 0.5 sec. after landing.

The exposure energy on the medium was adjustable in the range of 0.01 to 15 J/cm² by setting the distance from the medium and the transport speed.

Comparative Example 5

Magenta ink 17 was prepared in the same manner as for magenta ink 14 except that 3 parts by weight of 9,10-dibutoxyanthracene was added, and filtration was carried out using a filter having an absolute filtration accuracy of 2 pm.

A magenta image was produced in the same manner as in the method of Example 13 using magenta ink 17 prepared as above.

Evaluation of Inkjet Image

The images thus formed were subjected to evaluation in accordance with the methods described below in terms of sensitivity required for curing, penetration into commercial recycled paper, ink spreading on a grained aluminum support, adhesion, plate life, and storage stability.

Curing Sensitivity Measurement

The exposure energy intensity (mJ/cm²) when a feeling of tackiness disappeared on the image surface after irradiation with ultraviolet rays was defined as the curing sensitivity. The smaller the value, the higher the sensitivity.

Evaluation of Penetration into Commercial Recycled Paper

Images printed on commercial recycled paper were evaluated in terms of penetration in accordance with the criteria below.

-   Good: Hardly any penetration, no odor of residual monomer. -   Fair: Slight penetration, slight odor of residual monomer. -   Poor: Ink obviously penetrated to the reverse side, and strong odor     of residual monomer.     Evaluation of Ink Spread on Grained Aluminum Support

With respect to an image printed on a grained aluminum support, ink spread was evaluated in accordance with the criteria below.

-   Good: no spread between adjacent dots. -   Fair: dots slightly spread. -   Poor: dots spread and image was obviously blurred.     Evaluation of Adhesion to Grained Aluminum Support

With regard to the printed images formed above, a completely undamaged sample and a sample whose printed surface was crosshatched with 11 cuts in both lengthwise and widthwise directions at intervals of 1 mm in accordance with JIS K 5400 to give 100 of 1 mm squares were prepared, Sellotape was affixed to the printed surface of each sample and peeled off quickly at an angle of 90 degrees, and the condition of the remaining printed image or squares that had not been peeled off was evaluated in accordance with the criteria below.

-   Good: printed image was not peeled off at all in the crosshatch     test. -   Fair: the ink was slightly peeled off in the crosshatch test, but     unless the ink surface was damaged little was peeled off. -   Poor: easily peeled off by Sellotape (registered trademark) under     both conditions.     Evaluation of Plate Life

An image printed on a grained aluminum support prepared above was used as a printing plate, printing was carried out using a Heidel KOR-D machine, and a relative comparison of the number of prints completed was used as an index for the plate life (the number obtained for Example 1 was defined as 100). The larger the number, the longer the plate life, which is preferable.

Storage Stability Evaluation

After storing the prepared ink at 75%RH and 60° C. for 3 days, the ink viscosity at the. discharge temperature was measured, and an increase in the ink viscosity was expressed as a viscosity ratio (after storage/before storage). When the viscosity was unchanged and the ratio was close to 1.0, the storage stability was good, and if the ratio exceeded 1.5, clogging might undesirably be caused during discharge.

These evaluation results are shown in Table 4. TABLE 4 Ma- Curing genta sensi- Storage ink tivity Pene- stabil- Ink Ad- Plate No. (mJ/cm²) tration ity spread hesion life Ex. 1 1 100 Good 1.1 Good Good 100 Ex. 2 2 90 Good 1.1 Good Good 110 Ex. 3 3 110 Good 1.1 Good Good 90 Ex. 4 4 80 Good 1.3 Good Good 120 Ex. 5 5 110 Good 1.1 Good Good 90 Ex. 6 6 110 Good 1.1 Good Good 95 Ex. 7 7 120 Good 1.1 Good Good 85 Ex. 8 8 120 Good 1.1 Good Good 85 Ex. 9 9 70 Good 1.2 Good Good 150 Ex. 10 10 90 Good 1.1 Good Good 110 Ex. 11 15 90 Good 1.1 Good Good 110 Ex. 12 16 80 Good 1.1 Good Good 120 Ex. 13 9 70 Good 1.2 Good Good 150 Comp. 11 250 Fair 1.1 Fair Fair 60 Ex. 1 Comp. 12 200 Good 1.1 Good Fair 70 Ex. 2 Comp. 13 200 Good 1.1 Good Fair 70 Ex. 3 Comp. 14 200 Good 1.1 Good Fair 70 Ex. 4 Comp. 17 180 Good 1.2 Good Fair 75 Ex. 5

It is clear from Table 4 that the ink composition of the present invention comprising (a) an onium salt, the counteranion thereof having a volume of 1,000 A³ or greater, and (b) a cationically polymerizable compound has high sensitivity toward radiation, can form a high quality image in terms of image formation properties on paper, has good storage stability, and can form a high quality image with high plate life when applied to the production of a printing plate. 

1. An ink composition comprising: (a) an onium salt, a counteranion thereof having a volume of 1,000 A³ or greater; and (b) a cationically polymerizable compound.
 2. The ink composition according to claim 1, wherein the counteranion of the onium salt has a volume of 1,000 to 5,000 A³.
 3. The ink composition according to claim 1, wherein the counteranion of the onium salt is an organoboron anion.
 4. The ink composition according to claim 1, wherein the counteranion of the onium salt is an anion in which at least one perfluoroalkyl group is bonded directly to an anionic atom or is bonded to the anionic atom via an unsaturated bond or an aromatic ring.
 5. The ink composition according to claim 1, wherein the counteranion of the onium salt is an anion selected from the group consisting of An-1 to An-12 below.


6. The ink composition according to claim 1, wherein a countercation of the onium salt is a cation selected from the group consisting of iodonium, sulfonium, phosphonium, diazonium, ammonium, pyridinium, quinolinium, acridinium, oxonium, selenonium, and arsonium.
 7. The ink composition according to claim 1, wherein the ink composition comprises a colorant.
 8. The ink composition according to claim 7, wherein the colorant is a pigment or an oil-soluble dye.
 9. The ink composition according to claim 8, wherein the oil-soluble dye has an oxidation potential of 1.0 V (vs SCE) or greater.
 10. The ink composition according to claim 1, wherein it is for inkjet recording.
 11. An inkjet recording method comprising: (a′) a step of discharging an ink composition onto a recording medium; and (b′) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation, wherein the ink composition is the ink composition according to claim
 1. 12. The inkjet recording method according to claim 11, wherein the actinic radiation is ultraviolet radiation emitted by a light emitting diode that has a light emission peak wavelength in the range of 350 to 420 nm and generates ultraviolet radiation whose maximum illumination intensity on the surface of a recording medium is 10 to 2,000 mW/cm².
 13. A printed material recorded by the inkjet recording method according to claim
 11. 14. A process for producing a lithographic printing plate, the process comprising: (a″) a step of discharging the ink composition according to claim 1 onto a hydrophilic support; and (b″) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation so as to form a hydrophobic image on the hydrophilic support from the cured ink composition.
 15. A lithographic printing plate produced by the process for producing a lithographic printing plate according to claim
 14. 